Heterogeneous Ziegler-Natta TiCl 4 catalyst using MgCl 2 and SiO 2 as supports was prepared under controlled conditions. Mg(OEt) 2 was used as a starting material and was expected to convert to active MgCl 2 during catalyst preparation. Due to the high surface area and good morphological control, SiO 2 was chosen as well. Slurry copolymerization of ethylene and propylene (EPM) was carried out in dry n-heptane by using the catalyst system SiO 2 / MgCl 2 /TiCl 4 /EB/TiBA or TEA/MPT/H 2 at temperatures of 40 -70°C, different molar ratios of alkyl aluminum : MPT : Ti, hydrogen concentrations, and relative and total monomers pressure. Titanium content of the catalyst was 2.96% and surface area of the catalyst was 78 m 2 /g. Triisobutyl aluminum (TiBA) and triethyl aluminum (TEA) were used as cocatalysts, while ethyl benzoate (EB) and methyl ptoluate (MPT) were used as internal and external donors, respectively. H 2 was used as a chain-transfer agent. Goodquality ethylene propylene rubber (EPR) of rubber was obtained at the ratio of [TiBA] : [MPT] : [Ti] ϭ 320 : 16 : 1 and polymerization temperature was 60°C. When TiBA was used as a cocatalyst, a higher and more rubberlike copolymer was obtained. For both of the cocatalysts, an optimum ratio of Al/Ti was obtained relative to the catalyst productivity. Ethylene content of the copolymer obtained increased with increasing TiBA concentration, while inverse results were obtained by using TEA. Addition of H 2 increased the reactivity of the catalyst. The highest product was obtained when 150 mL H 2 /L solvent was used. Increasing temperature from 40 to 70°C decreased the productivity of the catalyst, while irregular behavior was observed on ethylene content. Relative pressure of P P /P E ϭ 1.4 : 1 and total pressure of 1 atm was the best condition for the copolymerization. Polymers with ethylene contents of 25-84% were obtained. Increasing ethylene content of EPR decreased T g of the polymer obtained to a limiting value. Viscosity-average molecular weight (M v ) decreased with increasing temperature and TiBA and H 2 concentration. However, increasing the polymerization time increased the M v .