This study examines whether the instability of self-esteem (i.e., a high intraindividual variability in self-esteem) is differentially associated with different types of aggressive behavior by using a sample of 235 preadolescent children. Self-esteem was measured four times for four consecutive days, and proactive and reactive aggressive behaviors were assessed by peer nominations. The latent trait-state-occasion model was used to describe the stable trait and the fluctuating state components of self-esteem. The empirical results indicated that the state component of self-esteem, not the trait component, had a significant association with aggressive behaviors. When controlling the trait component, the state component was positively related to reactive aggression and negatively related to proactive aggression. Implications for future research were discussed.