The hydration of cement containing ultra fine slag was studied and compared with the hydration of cement containing ordinary fine slag. The hydration production was analysed by TG DTA, XRD and SEM, hydraulic degrees were determined. The results show, hydraulic rate of cement containing ultra fine slag is much higher than that of cement containing ordinary fine slag. At early hydation, ultra fine slag can consumes a lot of Ca(OH) 2 in the paste and produces much of gel. The physical and mechanical properties of hardened cement paste are enhanced and the microstructure is improved.