The relationship between a student's educational major and his individual characteristics, personally traits and social behavior (Which in turn determine an individual student's success) make one think more seriously of the way students are selected for entry into higher education and further studies. The purpose of this study is to investigate the relationship between four variables (Cognitive entry behavior, personality traits, social origin and interest) as effective factors in agriculture student's selection and their educational success in his studies. The sample consisted of 655 students who had entered college of agriculture of Tehran University during the years 1990-1999. High school grade point average, the final grade point average, passing M.S. entrance exam, were selected as indicator of success in this study. The results showed that father's education, to be a more effective factor in school's success of agricultural students as compared with other factors in social origin. Fathers' residence (rural versus urban) and occupation (farmer versus non-farmer) did not effect school's success. Personality traits based upon NEO questionnaires did have any effect either. Interest in major did not contribute to students' success. This finding which is contrary to general finding by others' can be explained with regard to low interest in agriculture major by students. Among the entrance exam test subjects such as English as a second language, Arabic, Chemistry and Geology were more effective. Among the variables such as cognitive entry behaviors, social origin, interest and personality traits, cognitive entry behaviors (entrance exam) had more effect on B.S. grade points average and acceptance of the student for M.S. and Ph.D. degree programs.