Fly ash, which is a byproduct material of thermal power plants, is widely used to produce economic and durable concrete. The strength development of fly ash concretes depends on chemical and physical properties of fly ash and particle size. Curing conditions are also important on the strength development of fly ash concretes. In this experimental study, effects of nano-SiO2 on strength development of fly ash cement mortars were investigated. Mortars having three different fly ash replacement ratios, four different water to cement ratios, and three nanoSiO(2) addition ratios, were produced. Compressive and flexural strength tests were applied on mortar specimens at the end of 3,7,28 and 90 days of curing. The most important result of the study is that the nano SiO2 addition improved the early age (3 and 7 days) compressive and flexural strength properties of fly ash cement mortars. Moreover, for 3 and 7 day ages, the efficiency of nano SiO2 increased with increasing fly ash replacement ratio of mixtures.