Abstract: How the state and content of RE vary in high-carbon steel and affect its mechanical properties were studied experimentally and theoretically. It was found that adding RE properly to the steel can improve significantly and stably steel's mechanical properties even if the cleanliness of steel has greatly been enhanced and that the adding amount of RE can properly be reduced along with the increasing cleanliness. Experimental results showed that the plasticity and impact toughness can be improved greatly when the optimum adding amounts of RE are 0.02 and 0.01 wt% to low and high cleanliness steels, respectively. However, adding a large amount of RE can't increase solid RE solution content in low cleanliness steel, which will be kept below 0.0012 wt%.