The aims of this study are to investigate the effects of four curing conditions on the engineering properties of hardened slag-based mortar which was produced by the mixture of ground granulated blast furnace slag, Type F fly ash, circulating fluidized bed combustion Fly ash, and river sand. Four curing conditions, including three different continuous initial curing periods of 3, 7 and 28 days in a chamber with the constant temperature of 30oC and constant humidity 65 % RH, respectively, then in the air until the testing at 28 days, and the fourth one being a continuous water curing of 30oC until the testing at 28 days, were studied. The results showed that, for the water-to-binder ratio of 0.35, the 28-day compressive strengths of hardened mortar of curing in 30˚C and 65 R.H condition were much higher than those of air curing after initial curing periods of 3 and 7 days and continuous water curing of 30˚C by 12.4%, 8.94% and 8.83%, respectively. It was also noticed that a slightly increase of compressive strength of hardened mortar by a narrow range from 0.34% to 2.3% with water-to-binder ratio of 0.5 being curing for 28 days in the chamber of constant 30˚C and 65 RH as compare with those of other types of curing conditions. The compressive strengths of hardened mortar with lower water-to-binder ratio obviously increase with longer initial curing period in the chamber. On the other hand, the water curing led to a lower compressive strength of hardened mortar probably due to the calcium hydroxides of hydration products being gradually dissolved in water.