Abstract: In the copolymerization of ethylene and propylene, the catalytic efficiency of the Ziegler-Natta type V-Al catalyst system might be enhanced by adding some organic compounds as activators. With the addition of p-toluene sulphonyl chloride (TS), azobenzene (AB) and tetrahydrofuran (THF) as activators, the catalytic efficiencies of the V-Al catalyst system in both EPM and EPDM copolymerizations were raised and the amounts of vanadium compound used as main catalyst were reduced. Among these three activators, AB is most remakable. Catalytic efficiency of AB enhancements of up to 10 times as exhibited by the V-Al system alone were observed. The corresponding efficiency enhancements of TS and THF were 3-6 times and 1-2 times, respectively.The activation mechanisms of the polymerization mith the. addition of the activators were rarely reported except for the addition of trichloroa-cetate as activator. In the present paper, the activation mechanism is discussed in detail, the different effects of the added activators on the yields of the polymerization and the kinetic behaviour of the polymerization were studied. The reactions of these activators with the components of the V-Al catalyst system were also investigated. According to their modes of acti-vation, these activators could be divided into two groups. For TS and AB, the catalytic efficiency was improved through the regeneration of the deactivated centers by the oxidation of V²⁺ to V³⁺ , while for THF, through the prolongation of the life time of the active centers by the coordination with THF.