Interfacial transition zone (ITZ) constitutes the weakest link in the normal concrete matrix and significantly affects the properties of the concrete. To quantitatively evaluate the effect of ITZ on the physical and mechanical behavior of concrete, it is imperative to characterize the microstructure of ITZ. The intention of this paper is to investigate the characteristics of ITZ microstructure in a statistical manner In this paper an algorithm is developed to simulate the three-dimensional distribution of cement particles between aggregates in concrete.