The main purpose of this study was to examine the relationship among co-parenting, maternal parenting stress, and preschoolers' anxiety and hyperactivity with a sample of 155 mothers with 3 to 4 year old children (83 boys and 72 girls) living in Seoul. They completed a questionnaire on co-parenting, maternal parenting stress, and preschoolers' anxiety and hyperactivity. The results were analyzed by means of correlations and regressions. Co-parenting was positively correlated with preschoolers' anxiety and hyperactivity but affectionate, integrated co-parenting was negatively correlated with preschoolers' anxiety and hyperactivity. Maternal stress of parental suffering, dysfunctional interaction, and difficult temperament were positively related to preschoolers' anxiety and hyperactivity. Conflicting co-parenting was positively correlated with maternal stress of parental suffering, but affectionate and integrated co-parenting was negatively correlated with maternal stress of parental suffering and difficult temperament. Furthermore, maternal parenting stress mediated the relationship between co-parenting and preschoolers' anxiety and hyperactivity. Especially, maternal stress of parental suffering tended to play a perfectly mediating role between conflicting and integrated co-parenting and preschoolers' anxiety, between conflicting co-parenting and preschoolers' hyperactivity. Maternal stress of difficult temperament tended to play a perfectly mediating role between integrated co-parenting and preschoolers' anxiety and between conflicting co-parenting and preschoolers' hyperactivity. These results clearly indicate that maternal parenting stress plays a crucial role in the levels of preschoolers' anxiety and hyperactivity.