Dynamic mechanical behavior of low ethylene content propylene-ethylene copolymers was studied. It was found that there exists ethylene-propylene rubber component in the block copolymer, which is accordance with the component analysis. It is suggested that it is because of the existence of the ethylene-propylene rubber component that the block copolymer exhibits high impact strength. The relaxation temperature shifted to lower temperature for copolymers compared with homopolymer, and the damping peak was broadened by copolymerization, consistent with the increased chemical heterogeneity of the copolymers. Effects of thermal history and frequency on the relaxations were studied. The activation enthalpy for relaxation was determined.