In order to investigate effects of an electron donor in propylene polymerization(in gas and slurry phase reactions each) by using a 3rd generation Ziegler-Natta catalyst, the catalytic activities were measured before and after the addition of an electron donor. And physical properties of produced polymer were estimated. Polymerization rate was decreased with increasing EB/Al mole ratio because atactic sites were diminished. It was found that the overall reaction order was the 2nd order, and a reaction rate form was suggested by using a CO inhibition technique. Catalyst deactivation was due to the formation of dense isotactic polypropylene on the catalyst at the beginning of polymerization so that monomers had the difficulties in diffusion into catalyst surface through the dense polymer. From the measurements of polymer physical properties, probability of active site transitions from atactic→low isotactic→high isotactic sites could be considered, which may be due to the formation of a complex between co-catalyst and electron donor.