With blast-furnace slag is a by-product generated when pig iron is produced. It has been used as the concrete admixture due to high reactivity. However, It causes low strength development during early age. In order to make up for this drawback, in this study, we evaluated compressive strength of mortar replaced with high volume blast-furnace slag. Experimental results, Compressive strength of mortar based on blast-furnace slag is affected by cement type, substitution rate of blast-furnace slag and pH after mixing.