The furnace slag are the by-products of the steel industry, the main ingredients are the oxide of calcium, alumina and magnesium, and some silica. Slag used as concrete aggregate could cause the problem of the volume expansion of concrete. The expansion problem may be produced by ASR or free calcium oxide and magnesium oxide in slag. This research stabilizing the non-ASR reactive slag using steam treatment analyzes the effectiveness of steam treating technique inhibiting the expansive problem for slag used in concrete. And this paper also discusses the effect of the steam treating time on the performance of inhibiting the expansive problem.