A three-phase lattice model for predicting the elastic modulus of concrete is discussed. By simulating the mesostructure of concrete and applying a triangular lattice mesh with beam elements, a numerical method is presented for predicting the elastic modulus of concrete. The results show that based on reasonable consideration of the distribution of the aggregate, cement matrix and interfacial transition zone in concrete, the predictions from this model are in good agreement with the corresponding experimental results. Based on the model, the influence of the thickness, elastic modulus and Poisson's ratio of the interfacial transition zone on the relative elastic modulus of concrete is analyzed in detail.