This study investigates the effects of mother's playfulness, child's playfulness on social competence a mediated by self-regulation in child. Participants consisted of 281 child's mother and child's teachers. The mothers of all children completed the measures of 'Adult Playfulness Scale'. The teachers completed the measures of 'Children Playfulness Scale', 'Social Competence Rating Scale for Young Children', and 'Self-Regulation Scale for Young Children'. The results were as follows. First, child's playfulness and self-regulation were positively correlated with the child's social competence; however mother's playfulness was not correlated with child's social competence. Second, the results of the regression analysis revealed that mother's playfulness was not influenced child's social competence. Third, child's playfulness was influenced child's social competence. Forth, child's self-regulation was influenced child's social competence. Fifth, the results of the regression analysis revealed that child's playfulness was influenced child's social competence both directly and indirectly; child's playfulness had a direct effect on child's social competence and also had and indirect effect on mediated by self-regulation. The results indicated that self-regulation plays a crucial variable in the relationship of child's playfulness and child's social competence.