A comparative study has been made on the behaviors of ethylene, propylene, 1-butene and styrene polymerizations using a new MgCl2-supported TiCl4 catalyst prepared by the reaction of Mg(OEt)2 with Ti(OBu)4, phthalic anhydride and TiCl4, in conjunction with AlEt3 and phenyltriethoxy silane (PTES). Solubilities of monomers to a solvent (n-heptane) were also measured with a view to comparing the productivity of catalyst for each monomer and calculating the propagation rate constant (kp ) more exactly in slurry-phase polymerizations. The order of activity for olefin polymerizations was changed before and after normalization based on the concentration of monomer in n-heptane. The isotacticity (I.I.) of polypropylene (PP) and poly(1-butene) (PB) was remakably improved by PTES and simlar rate-time profiles were observed for propylene and 1-butene polymerizations, suggesting that the nature of active sites for both monomers is similar to each other. From the measurement of the number of active sites (C*), it was clear that both C* and kp were in the order of ethylene > propylene > 1-butene. The results on styrene polymerizations (activity and isospecificity) in the absence and presence of PTES suggested that isotactic polymerization of styrene proceeds via heterogeneous Ziegler-Natta catalysts without regard to the I.I. in propylene polymerizations.