As one of the mineral admixtures, silica fume (SF) with high pozzolanic reactivity is generally used to improve the properties of concrete. Most commercial silica fume used in concrete project is dry densified silica fume (DSF). It consists of a good deal of agglomerates of sizes ranging from 10 μm to several millimeters. The size of the undispersed silica fume is usually larger than that of the cement particle. At present, the effect of DSF on the hydration of cement is lack of studied. In this study, the commercial dry DSF is sifted into three sections (>150 μm, 80 ~ 150 μm, 35 ~ 80 μm) by different sieves. The amount of densified silica fume in mixture is 5% by weight of total binder. The used water/binder ratio (w/b) is 0.5. The effect of the fineness of DSF on the setting time of cement paste is firstly discussed. The hydration process of cement incorporated with different fineness of SF is investigated by a semi-adiabatic calorimetric for 3 days at 20 °C and x-ray diffraction (XRD). The obtained results indicate that the setting time of cement paste increases with the decrease of the fineness of DSF. At the w/b ratio of 0.5, the addition of SF has little effect on the hydration period of cement paste. But the hydration rate of cement is decreased with the increase of the size of DSF particles.