The aim of this study was to test the relationship of theory of mind (ToM) and maternal emotional expressiveness with children's aggressive behaviours (i.e., relational aggression, physical aggression). ToM is vital to relational aggression, which involves harming others through purposeful manipulation and damaging peer relationships. However, little is known about how ToM and family emotional environment are interactively associated with aggressive behaviours. Fifty-one Japanese children, aged 4 and 5 years, completed measures of ToM and receptive vocabulary. Mothers reported their emotional expressiveness towards their children and classroom teachers rated each child's aggressive behaviours. The results revealed that when boys have mature ToM and their mothers show high negative emotional expressiveness, they exhibit higher relational aggression in their peer relationships. Such interaction effect was not found for girls. The findings suggest that boys with mature ToM are likely to learn to use ToM for antisocial purposes via negative family emotional environments.