Objective: The goals of this research were (1)to examine the relationship among parental conflict, mothers depression and preschoolers sleep problems, and (2) to examine the mediating effects of mothers depression between marital conflict and preschoolers sleep problems. Methods: The participants were 1,639 mothers with preschoolers taken from the 5th year Korean Children Panel. Date were analyzed using descriptive statistics, partial correlation analysis, and multiple regression analysis. Results: The results were as follows: First, preschoolers sleep problems were positively related to marital conflicts. Second, preschoolers sleep problems were positively related to mothers depression. Third, marital conflict was positively related to mothers depression. Lastly, mothers depression fully mediated the relationship between marital conflict and preschoolers sleep problems. That is, marital conflict had an effect on preschoolers sleep problem, only through mothers depression. Conclusion/Implications: Based on the results, intervention programs should be developed to prevent preschoolers sleep problems such as marital relationship improvement programs and mothers depression improvement programs.