This paper focuses on the discrete modeling of triaxial behavior of concrete. The model developed for simulating the response of concrete specimens takes into account the heterogeneity at the mesoscopic scale. Behaviors of mortar, rock, and their interaction are identified a priori, by means of experimental tests on the mortar and the rock. The construction method of the discrete element assembly is based on the 3D segmentation of tomographic images. Such a method allows the modeling of concrete at the mesoscopic scale with an internal structure similar to the one of the concrete tested experimentally. The comparisons between numerical and experimental results show the model is capable to reproduce the triaxial behavior of concrete for confining pressure varying from 0 to 650 MPa.