The relationship between the late-age hydration and strength development of cement-slag mortars have been investigatedby measuring the compressive strengths and the non-evaporable water contents. The results show that the late-age strengthincreases with increasing the slag content. Increasing the fineness of slag makes greater contribution to the late-age strengthimprovement at high water to binder ratio than that at low water to binder ratio. At lower water to binder ratio, theincreasing rates of compressive strength and non-evaporable water content are smaller. There is a linear relationshipbetween the increasing rate of compressive strength and the increasing rate of non-evaporable water contents. The slope isalmost the same for all the samples at constant water to binder ratio and decreases with decreasing the water to binder ratio.