Abstract: An interfacial spring element model (ISEM) is employed to simulate the strength of concrete under uniaxial compressive loading. In this model, the concrete was considered as a composition consisting of aggregate, matrix, interfacial transition zone (ITZ) and initial defects in ITZ at meso level. The distribution and contents of the initial defects, and the ITZ properties parameters are discussed, accordingly which, the range of meso-parameters are suggested. The results show that the compressive strength of concrete decreases with the increase of the ITZ initial defects. When the initial defects in ITZ achieves to 30 percent of ITZ elements, the numerical simulation results of concrete compressive strength agree well with the experimental results.