This study explored the relationships between parenting behavior, preschooler's effortful control, and social adjustment. The subjects were 351 preschoolers(174 boys, 177 girls) and their parents. The major results of this study were as follows. First, there were significant relationships among the factors analyzed in this research, namely, affectionate parenting behavior, preschooler's effortful control, and social adjustment. Second, preschooler's effortful control mediated the effects of affectionate parenting behavior upon preschooler's social adjustment. In conclusion, this study revealed that parents' affectionate parenting behavior and a preschooler's effortful control make a significant impact on the level of the preschooler's social adjustment. Based on th findings of this investigation, the implications of the preschooler's effortful control were also discussed.