Blended cement pastes were prepared with and without silica fume and fly ash using an initial water/cement ratio of 0.40. Effect of these two mineral admixtures (silica fume, and fly ash) on the heat evolution of hydration reactions of Portland cement produced in Yemen, Egypt and Saudi Arabia has been studied at various hydration times. The heat of hydration of OPC in the presence of admixtures was studied, by using calorimetric method. The results show that the heat of hydration of Portland cement is retarded in the presence of both the admixtures