This paper tests the influence of limestone powder (LsP) on the hydration of cement mineral by XRD and SEM,and studies the influence of LsP on mortar workability and mechanical properties systematically.The results show that LsP has little influence on the hydration C3S.The reaction between LsP and C3A could occur at the early age and induce 3CaO?Al2O3? CaCO3?11H2O,which can exist steadily at the later age.The increase of LsP fineness or replacement level can promote the reaction.LsP has little influence on the mortar fluidity,and the mortar compressive strength is decreased by LsP.