Disclosed is a catalyst composition for use in the polymerization of olefins, which is comprised of (a) a catalyst component containing magnesium, titanium, a halogen, and an ingredient derived from a keto-ester compound, and (b) an organic aluminum compound. The catalyst activity and capability of providing a highly stereoregular polymer are enhanced by preparing the catalyst component (a) by a process wherein, during or after the formation of a solid catalyst component containing magnesium, titanium, and a halogen, the solid catalyst component is treated with a keto-ester of the formula: ##STR1## wherein R.sup.1, R.sup.2 and Z represent a hydrocarbon group.