The purposes of this study were to explore the effect of mother-child emotional availability on preschooler"s preschool adjustment and mediating effect of preschooler"s effortful control on the relationship between mother-child emotional availability and preschooler"s preschool adjustment. 222 preschoolers(110 boys, 112 girls, aged 5) and their mothers participated in this study. Mother-child emotional availability was assessed by Emotional Availability Scale(Biringen, et al., 1998). Preschooler"s effortful control was measured by the Child Behavior Questionnaire(Rothbart, et al., 1994). Preschooler"s school adjustment was measured by Teacher Rating Scale of School Adjustment: TRSSA(Buhs & Ladd, 2001). Data were analyzed using descriptive statistics, correlations, and structural equation modeling analysis. The major results of this study were as follows; First, Mother-child emotional availability effected on preschooler"s preschool adjustment. Second, Mother-child emotional availability effected on preschooler’s effortful control. Third, Preschooler’s effortful control effected on preschooler"s preschool adjustment. Forth, preschooler"s effortful control mediated the effect of mother-child emotional availability on preschooler"s preschool adjustment. That was, the more mother-child were emotionally available to each other, the more preschooler"s effortfully control their attention and behavior, which in turn contributed to preschooler"s preschool adjustment. In conclusion, mother-child emotional availability affect on preschooler"s preschool adjustment and preschoolers control mediates the relationship between mother-child emotional availability and preschool adjustment.