The effect of fly ash and nanoCaCO3 on the viscosity of pastes was studied. The rheological value of cement paste was determined by the rotation rheometer NXS-11B. In the study, five different dosages (0%, 20%, 30%, 40%, and 50%) of fly ash and three levels of nanoCaCO3, 0.5%, 1%, and 2.5%, were considered. Viscosity of the pastes, made with fly ash and nanoCaCO3 at a constant water-to-binder ratio of 0.35, were measured and analyzed. The results indicate that the pastes with fly ash or/and nanoCaCO3 still fit the Bingham model. The addition of fly ash reduced viscosity, however, the addition of nanoCaCO3 increased viscosity. The effect of nanoCaCO3 is more significantly than fly ash on viscosity.