Abstract: The effects of the catalyst system and composition of ethylene-propylene copolymers on the crystallization rates of the copolymer have been studied by dilatometry. It is shown that for the same catalyst system, the maximum crystallisation rate 1/t_(1/2)(max) of the copolymer increased with the increase of the ethylene content in it.The maximum crystallization rates of ethylene-propylene copolymers obtained with various catalyst systems were found in the following order:(VOCl₃-) >(V_5-0) >(V(_acac)₃-) The order of 1/t₁/_2(max) for the catalyst system with addition of an active agent ETCA was:(VOCl₃-ETCA) >(V_5-9-ETCA) >(V(_acac)₃-ETCA) The results of the crystallinity and glass transition temperature of the coplymers, as well as the composition of copolymers of differnt sequence suggested that the ethylene-propylene copolymer obtained with VOCl₃- had longer PE species and those obtained with V_5-9- had shorter PE species. More irregular structure was formed when the copolymer is obtained with V_(acac)3-catalyst system.