The purpose of this study was to examine the effects of mothers’ parenting behavior, preschool children’s language ability and emotion regulation on peer competence and aggressive behavior. The subjects were 100 preschool children (49 girls and 51 boys; mean age, 70.30 months), their mothers and teachers, recruited from five daycare centers in located in Gyeonggi-do area. Each child’s language ability was assessed individually with the standardized measure, the Preschool Receptive-Expressive Language Scale and their teachers reported on the children’s peer competence and aggressive behavior. Their mothers also reported on parenting behavior and their child’s emotion regulation by questionnaire. The collected data was analyzed by correlation analysis and hierarchical regression. The main results of this study were as follows. First, preschool children’s peer competence was positively related to maternal warmth and children’s language ability. Their aggressive behavior was positively related to harsh maternal parenting but negatively related to emotion regulation. Second, hierarchical regression analyses revealed that children’s language ability and maternal warmth predicted peer competence. Third, children’s gender, emotion regulation, and harsh maternal parenting predicted aggressive behavior. These findings could provide basic information for programs and services to promote peer competence in preschool children.