This study investigated the thermal resistance of Alkali Activated Slag (AAS) concrete. Alkali Activated Slag cement is an environmentally friendly alternative to Portland cement, which can be produced by using an alkaline solution to activate the binding effect of the blast furnace slag. Heat from fire changes the physical and mechanical properties of concrete. Given the decisive role of thermal resistance in the operation and performance of structures, it is necessary to evaluate the effect of heat on the performance of AAS concrete. In this study, a series of tests were arranged to examine the effect of slag percentage on thermal resistance of AAS concrete. AAS concrete samples from mixes, with different slag percentage, were subjected to 20, 200, 400, 600, and 800˚C temperature and change in their compressive strength were measured and compared with that of samples made by ordinary Portland cement concrete. The results show that the thermal resistance of AAS concrete is higher than ordinary concrete.