Background: Relatively few studies have investigated the effects of diet on behavior problems among preschoolers, particularly, physical aggression. In addition, children raised by poorly educated mothers usually have a higher probability of developing negative outcomes. Additionally, highly educated mothers have a higher probability of providing more healthy foods for their children. Thus, mothers providing healthy foods might mitigate children's behavior problems. The study aims to examine whether preschoolers' dietary pattern, as a manipulable factor, mediates the association between maternal education level and physical aggression. Methods: Data came from the Taiwan Birth Cohort Study (TBCS), a nationally representative population-based cohort study, which included 18,513 five-year-old Taiwanese children. Mothers and primary caregivers reported the information on preschoolers' physical aggression and food consumption at age 5 and maternal education level at age 6 months. Two dietary patterns, namely a healthy diet and a high-fat-sugar-salt (HFSS) diet, were retrieved by exploratory factor analysis. Mediation hypotheses were tested by a series of multiple regression models conducted using the PROCESS macro of SAS 9.4. All models were adjusted for children's sex, parental marital status, household income, mental distress at age 5 and children's physical aggression at age 3. Results: Maternal education positively linked to healthy dietary patterns (B = 0.014, p = 0.002) which was negatively associated with preschoolers' physical aggression (B =-0.096, p = 0.013), and it is negatively related to the HFSS dietary pattern (B =-0.042, p = 0.002) which was directly positively associated with preschoolers' physical aggression (B = 0.123, p = 0.008). The association between maternal education and preschoolers' physical aggression was partially mediated by preschoolers' healthy (B =-0.001, p < .001) and HFSS (B =-0.005, p = <.001) dietary patterns, respectively. The R-square of the mediation model is 0.178.