The purpose of the study was to investigate the differences between sub-factors of stress and social support, to analyze the differences of stress and social support in terms of gender, and to examine the role of social support as a mediating variable in the relationship between stress and academic achievement. Subjects were 3,449 middle school students. Female students were 1,724, and male students were 1,725. The instrument used in this study was from Korea Youth Panel Survey of KYDI. From the instrument, 1 academic achievement question, 17 stress questions, and 112 social support questions were used. For the analysis of data, t test and optimal scaling regression were used. The results of the study were as follows. First, youths experienced more study related stress than other kinds of stress. Second, youths perceived more peer social support than other kinds of social support. Third, female adolescents experienced more study and appearance/money stress than male adolescents. Fourth, female adolescents perceived more parents social support than male adolescents, while male adolescents perceived more peer and teacher social support than female adolescents. Fifth, social support played a role of complete mediating variable in the relationship between stress and academic achievement. Sixth, in order to improve youth\'s academic achievement, it is needed to pay attention to social support.