isotactic-Polypropylene (PP) composites reinforced with 20, 45, and 60 wt% fly ash (FA) were prepared by injection molding at 483 K. The thermo-mechanical and morphological behaviors of PP and composites were examined. The addition of FA led to significant enhancement of both storage (E') and loss modulus (E'') of the composites compared to those of PP at a given temperature, supporting the view of the formation of an interfacial interaction between the surfaces of PP and FA. By increasing the temperature from 303 to 423 K, these properties decreased exponentially, and indicated the temperature-activated mobility of segmental chains of PP. The tangent δ curves of PP and composites showed broader patterns. The glass transition temperature (Tg) was assigned at 343 K for PP and 245-246 K for the composites. In the theoretical calculation of the storage modulus and tangent δ, the modifications were required in the existing equations to fit the experimental data well. Scanning electron microscopy (SEM) revealed a uniform distribution of spherical FA particles in the composites. The calculated interfacial interaction between the FA and PP surfaces was 1.85, which is similar with other composite systems.