Interfacial transition zone (ITZ) has long been of particular interest in concrete technology. The limited sensitivity of experimental techniques makes it attractive to study ITZ using computer simulations. In this paper, a multi-aggregate approach is proposed to simulate the formation of ITZ in concrete. In light of a modified status-oriented computer model for simulating cement hydration, the evolution of ITZ is also simulated in this approach. Through simulations, the influences of several factors related to concrete mixture proportion on ITZ are investigated. It is found that ITZ thickness, as defined by the overall average porosity, can be reduced by using finer aggregate, increasing aggregate volume fraction, reducing water-cement ratio (w/c), or making the binder system finer. Following hydration, the ITZ thickness decreases continuously, but the difference of porosity between ITZ and bulk paste keeps almost constant at mature ages.