This persent study was designed to examine the moral reasoning ability of persons who have different age, level of education and length of post education period. There were four major research objectives. First, to examine the moral reasoning ability of persons of the same age level who have different levels of education. Secondly, to investigate the moral reasoning ability of persons who have different levels of education and different length of post education period but are within the same age level. Thirdly, to compare the amount of relationship between the moral reasoning ability and the three basic psychological  traits with  the relationship between the 
moral reasoning ability and each of these three paycological traits. Finally, to compare the amount of variances of the moral reasoning ability accounted for by the following independent variables, the three psycological traits, five bio-social characteristics and background variables, and the mass media usage, combine or seperated into 3 groups. 
                The samples in this study consisted of 437 students and non-students in Bangkok, whose ages ranged from 10-50 years. all of these respondents have completed prathomsuksa 6 and mathayomsuksa 6 within 1 year or would complete at the end of the year, some of them have finished prathomsuksa 6 and mathayomsuksa 6 for more than 4 years under formal education or non-fermal education. 
                In this field study, the respondents were asked to respond to sets of questionnaires. There were 4 sets of variables being assessed. (1) level of the moral reasoning assessed by two types  of questionnaires namely, the objective moral reasoning test and the Defining IssusesTest  (D.I.T.). The Defining Issuses Test generated 3 types of scores; D.I.T. 1 (sum of weight ranks given to all stage 5 and 6 items), D.I.T. 2 ( sum of weigned ranks given to all stage 4 items), D.I.T. 3 (sum of  weighted ranks given to all stage 2 and 3 items) (2) The three basic psychological traits of the moral reasoning development were assessed by the tests of Thai language skills, mental health and empathy. (3) The bio-social characteristics and background variables of the respondents were assessed by background questionnaire. (4) The mass media usage was assessed by a test. 
              Four hypotheses were tested this study by carrying out the Analysis of Variance with two-way Factorial Designs, and the Stepwised Multiple Regression Analysis on the data of the total sample, as well as on the data of different groups of respondents, classified by the bio-social and background variables. 
                There were four major findings in this study. 
                First, it was found that in the respondents who were younger than 25 years old with low education, the persons whose post eduction period was longer (more than 4 years) gave more importance to stage 4 reasoning than the persons with shorter period of post education (not more than one year). In the older group (more than 24 years old), the low-education persons with long post period and the high-education persons with short post period gave more importance to stage 5 and 6 reasoning than the low-education persons with short post period. This part of the result pointed to the fact that there may exist a critical period during which individuals should receive an appropriate level of education witch could easily induce moral reasoning development in the receivers. This in turn could lead to further development of moral reasoning when they get older. 
                Secondly, it was found that in the younger group, the highly educated persons gave more impertance to stage 5 and 6 reasoning than the lowly educated persons. At the same time the lowly educated persons gave more importance to stage 2 and 3 reasoning than the highly educated persons. The same results were also founded in the total sample, the male group, the female group, the high economic status group and the student group. In the older individuals (more than 24 years old), it was not evident that whether the individuals with different level of education would differ on moral reasoning ability. 
              Third, it was found that Thai language skill, mental health and empathy each could in turn predict moral reasoning ability,  but together they could not. However it was found that the respondents who had high scores on all three tests of basic characteristics  (Thai language skills, mental health, empathy) stressed the importance of stages 5 and 6 of moral reasoning more strongly than the respondents who had low scores on all these three tests. 
                Finally, it was found that the three basic psychological traits, the bio-social characteristics and background variables and the advantageous mass media usage together predicted moral reasoning ability more than only the three psychological traits could or only the bio -social characteristics and background variables could. 
                This field study is able to show the importance of providing suitable education during the critical period which can facilitate the development of moral reasoning ability in the individuals which can became good basis for further development at an older age. In addition this study show the long-termed effects of Thai formal education which could facilitate the development of moral reasoning ability of the individuals.