A phthalate-free catalyst component for olefin polymerization comprising titanium, magnesium, a halogen, diether, and oxalic acid diamides represented by the following formula (I): and an olefin polymerization catalyst system consisting of the solid catalyst component, an organoaluminum compound, and an optional external electron donor compound. The present catalyst compositions improve diether-based catalyst selectivity while maintaining excellent catalyst activity and hydrogen response. An olefin polymer that has a moderate molecular weight distribution while maintaining higher stereoregularity (isotacticity) over diether-based catalyst, and better than or equal to phthalate-based catalyst can be produced by utilizing the inventive catalyst component.