The purpose of this study is to examine the effects of attachment experience in childhood and parental reflective functioning on parenting stress of mothers with toddlers and preschoolers. Special attention is given to whether or not the relationship between mothers’ attachment experience and parenting stress is mediated by parental reflective functioning. Four hundred and eighty mothers participated in the study via an online survey. Descriptive statistics and Pearson’s product correlation coefficients were obtained, and tests of the mediation were performed by structural equation modeling. Significant correlations were found between mothers’ attachment experience and parenting stress, between mothers’ attachment experience and parental reflective functioning, and between parental reflective functioning and parenting stress. While the mediation effects of mothers’ parental reflective functioning were found to be statistically significant, the overall fit of the research model was not found to be sufficient. Although the model fit was less than optimal, the results obtained in the path coefficients seem to indicate that enhancing mothers’ understanding of child’s mental states and appropriate behaviors in parenting context could reduce parenting stress.