The theory of micromechanics of composite materials and three dimensional microstructure model were used to develop a multiphase model of micromechanics which is capable of simulating the elastic properties of cement pastes. The hydrated products, unhydrated cement and water(in capillary pores) in the cement paste are seen as matrix, inclusion and equivalent medium respectively. The evolution of elastic properties in the hydration process was calculated at different watercement ratio. The parameters of the proposed model are the mineral composition and the intrinsic properties of the constituent components of cement pastes. Compared with the experimental results, this model can be used to predict the elastic properties of cement paste.