The purpose of this study was to explore the mediating effects of preschooler`s effortful control on the relationship between maternal emotional availability and preschooler`s social skills and problem behaviors. One hundred-thirty six 5-year-old preschoolers and their mothers participated in this study. Instruments for this study were the Emotional Availability Scale for maternal emotional availability, the Delay task, and the Child Behavior Questionnaire for preschooler`s effortful control, and the Social Skill Rating Scale, K-CBCL 1.5-5 and K-TRF for preschooler`s social skills and problem behaviors. The resulting data were analyzed using descriptive statistics, partial correlation, and structural equation modeling analysis. As predicted, the preschooler`s effortful control mediated the effects of maternal emotional availability on preschooler`s social skills and problem behaviors. In conclusion, the preschooler`s effortful control mediates the effects of emotion related socialization behavior on the preschooler`s socio-emotional adjustment.