We investigated what a dyadic framework added to Eisenberg, Cumberland, and Spinrad's (1998) parental emotion socialization model based on the argument that the dynamic organization of emotion in the dyad is more than the sum of its parts and thus makes a unique contribution to emotion socialization. Preschoolers (N = 235) completed challenging problem-solving tasks with mothers and fathers, during which parental emotion-related socialization behaviors (ERSBs), child negative emotional arousal, and dyadic positive emotion data were collected. We examined whether dyadic synchrony of positive emotion at age 3 was a mechanism by which age 3 parental ERSBs impacted children's age 5 aggressive behavior in school, accounting for child gender, child negative emotional arousal, and aggressive behavior in preschool. ERSBs were significantly positively related to dyadic positive synchrony with both mothers and fathers at age 3. Longitudinal models supported an indirect effect, not a moderating effect, of dyadic synchrony: both mothers' and fathers' ERSBs contributed to children's less aggressive behavior at age 5 through the effects of higher dyadic positive synchrony. Findings suggest dynamic, dyadic emotional processes should be considered as a mechanism of emotion socialization and that parent-child positive emotional synchrony is supportive of early childhood emotional development.