The effect of two kinds of mineral admixtures (MA) on volume stability of steel slag mortar (SSM) is investigated in order to solve the expansion problem due to steel slag. The rate of linear expansion and flexural strength of SSM containing mineral admixture have been compared with control specimens. Addition of FA and GGBFS slowed down the rate of expansion gain and prolonged the stable ages of SSM. In addition, FA and GGBFS increased later flexural strength of SSM. The improving effect of FA and GGBFS on volume stability of SSM was attributed to the increase of later flexural strength and enhancement of compaction rate.