Abstract: In this article stearic acid was used to modify steel slag in the planetary mill with average particle size and activation index as token parameters. The change of activation index of modified steel slag was studied in order to find the best way to modify the slag. The activation index of the steel slag reached the highest level when ground for 20 minutes in the planetary mill with 0.5% (weight rate) of stearic acid .Infrared spectrum analysis revealed that the stearic acid and steel slag had combined in chemical bond.