Abstract: The action of organomagnesium compounds on promoting the catalytic efficiency of the highly active catalyst of Ti system with MgCl₃ as carrier for polymerization of ethylene has been studied. It was found that the promotive effect increases with the content of MgCl₃ in the composited carrier. The scanning electron micrographs of several MgCl, supported catalysts before and after treatment with organomagnesium compound show that morphology of the treated catalyst displays globular micrograin cluster which is finer and more porous than that of the untreated one. It has been suggested that the promotive action given by organomagnesium compounds includes raising porosity of the carrier, as well as electronic effect of the modifying ligand's towards the active center Ti⁺³.