Both compressive strength and appearance of the alkali activated slag cement paste and the alkali fly ash slag cement pastes with different fly ash contents were investigated under the hot environment from room temperature to 1000℃. The phase composition as well as the microstructure of these cement pastes were observed by using XRD and SEM. Results indicate that both the compressive strength and high temperature resistance of alkali activated slag cement pastes are better than that of alkali fly ash slag cement pastes. At 600℃, alkali activated slag cement paste has shown a phase reaction that leads to the formation of gehlenite. The alkali fly ash slag cement pastes with 50% and 30% (by mass) of fly ash lose their compressive strength completely at 800℃ and 1000℃ respectively; a solid phrase reaction is found to form gehlenite and diopside at 1000℃.