The expansion of electric furnace reducing slag is delayed by its complex hydration. Before using the slag, it is necessary to stabilize the expansion property. Currently the reducing slag is used after aging for stabilization of expansion and judging its stability by the expansion ratio based on the standard expansion test results. Because of delayed expansion, a long time of aging or expansion test is required and then it is unable to judge exactly the degree of stabilization for expansion. In this report, a rapid stabilization method and an expansion test method using steam for reducing slag are presented on basis of some experimental results.