The purposes of this study were to examine relations among attachment security, playfulness, and social competence during the preschool period and to investigate relative effects of attachment security and playfulness on preschoolers’ social competence. A total of forty eight 4-year-old preschool children participated in the current study. Attachment security was assessed using the Attachment Q-sort during a two- or two and half hour home visit. Preschoolers’ playfulness and social competence were evaluated by mothers. First, preschoolers’ attachment security was significantly associated with their interpersonal adjustment. Preschoolers with secure attachment were rated by their mothers as having higher interpersonal adjustment. In addition, preschoolers’ playfulness was significantly related to popularity/leadership and social participation. Second, when preschoolers’ attachment security and playfulness were entered together to predict social competence, attachment security and sense of humor were significant predictors of preschoolers’ interpersonal adjustment, and social and cognitive spontaneity significantly predicted popularity/leadership. With respect to social participation, preschoolers’ physical spontaneity was a significant predictor. Findings of this study indicate that attachment security and playfulness tap into different aspects of social competence, suggesting that both are needed to be emphasized in the development of social competence during the preschool period.