Abstract: There is an interfacial transition zone (ITZ) between the coarse aggregate and mortar of concrete. The mechanical properties of ITZ are affected by the shape, size, content, spatial distribution and initial cracks of the aggregate. In order to study the influence of the ITZ strength on the properties of concrete materials, this paper used the discrete element software PFC2D to establish a concrete model with random aggregate distribution, and conducted uniaxial compression tests on it. It was found that when the strength of transition zone increased, the peak stress and crack initiation strain of concrete gradually increased, and the total number of cracks decreased. At the same time, when the strength of the transition zone increased, the failure type changed from shear failure to tensile failure, and the increase in the strength of the transition zone would transfer the crack initiation area of concrete from the transition zone to the mortar accumulation area. The simulation results can provide certain reference for the theoretical study on the ITZ in concrete.