This study was an attempt to investigate the effect of slag content on the compressive strength of river sand (RS) and oyster shell aggregate (OS) mortar in different aspects; fineness of slag, different percentage of slag content, cement types and use of chemical admixture (super plasticizer). Compressive strength of moist cured specimen was measured at 3, 7, 28, 91 days. Slag could be used up to 85% in partial replacement of cement and 75% slag content will give the highest strength for OS mortar. The compressive strength of RS-slag mortar could be lowered at initial stage but finally will increase about 17% than that of no slag mortar. OS mortar strength increased by adding chemical admixture with or without slag condition. Oyster shell aggregate could be used as fine aggregate and slag could be added with it to increase the compressive strength.