Abstract: The theories of mechanics of composite materials and poromechanics were used to develop a micromechanics model capable of simulating the effective elastic properties of Portland cement pastes. Using different length scales, four composite media could be identified for cement pastes and they were simulated using different modeling schemes: the C-S-H matrix was considered as a saturated porous medium; the hydration products were modeled using the Mori-Tanaka scheme; the effective elastic properties of the skeleton of cement paste were described using the three-phase model (or the generalized self-consistent scheme model); and finally the drained and undrained elastic moduli of cement pastes were simulated using the Mori-Tanaka scheme and the theory of poromechanics. The inputs of the proposed model are the intrinsic properties of the constituent components of cement pastes and can be easily obtained in the literature. The proposed model was used to predict the experimental results in the literature and the results demonstrated the effectiveness and accuracy of the model.