Stereoblock structures are found in the poly(propylene) fractions of low isotacticity produced by supported Ziegler-Natta catalysts. The influences of electron donors on the length of stereoblock and AlEt3 on the isotactic index of polymer were investigated. The formation of stereoblock structures is correlated to the interaction between AlEt3 and active sites. The nature of the active sites which produce different stereostructures and the reason that they can interconvert are explained with the concept of equilibrium. When electron donors are present in the catalysts, electron donors may exist in the environment of aspecific active sites and can be extracted by AlEt3, and these reactions are reversible. In the case of catalyst containing no electron donor, AlEt3 can complex with active sites, and there is an equilibrium reaction between monometallic and bimetallic active sites. These equilibrium reactions are responsible for the formation of stereoblock structures.