Partial replacement of cement with blast furnace slag has many advantages such as the reduction of construction fee, the decrease of hydration heat and the increase of long-term strength. This study investigates the effect of the fineness of slag on the rheological properties of cement paste and mortar. Slags with three different fineness were prepared, and they were used as partial replacement of cement 40% by cement weight. Test results showed that the partial replacement of BS1 and BS2 decreased flow and increased shear stress, whereas that of BS3 had an opposite effect, i.e., increased flow and decreased shear stress. Overall, in both cement paste and mortar conditions, it was found that the fineness effect of slag on the rheological properties was similar.