Some oxides in cement such as MgO, free CaO and SO3 may cause expansive reactions with time that lead to a decrease in concrete compressive strength. If such oxides were presented in high percentages, they may cause ultimate destruction of concrete. The present study was carried out to investigate the combined effect of MgO and SO3 contents in cement on compressive strength of concrete. Concrete mixes, with different MgO and SO3 percentages in type I and V cements, were cast. The results showed that there is a considerable effect of MgO content on concrete strength and on the optimum gypsum content in cement. The increase in MgO content results in a decrease in the compressive strength and a reduction in the optimum gypsum content. Type V cement appears to be more sensitive to the increase in MgO and SO3 contents than Type I.