Abstract: In this article the grinding-aid effect of two modifiers LYF and GM is studied on the steel slag, The change of the particle density with grinding time is examined and the morphology of the particle is observed under SEM. It is found that the modifiers have good grinding-aid effect on the steel slag and grinding efficiency reaches the highest at 1h grinding time in the planetary mill with 3mg LYF. The density changes with the grinding time in three stages.