The effects and mechanism of cerium, lanthanum and cerium-lanthanum alloys on microstructure and the impact toughness of high-carbon steel were studied in the present work. For high-carbon steel, the state and the content of RE were measured, and the effects and the mechanism of RE on sulfide inclusions, microstructure and the impact toughness of steel were determined. With increasing the RE addition, the sulfide inclusion can be changed from strip-like, spindle, ellipsoidal and spheric in shape. Strip-like sulfide disappears in high-carbon steel with the RE addition being 0.05%. A small quantity of RE can fine the austenitic grain, decrease the height of the SKK peak of the internal friction curve, and improve the impact toughness of high-carbon steel evidently. And the effects of lanthanum on fining the austenitic grain and improving the impact toughness is the largest, next to that of cerium-lanthanum alloys, and that of cerium is the least, which can be verified by the internal friction experiments.