Abstract: The toughening mechanism of concrete with different aggregate volume fractions was investigated, and the meso-mechanical model of mode II fracture toughness K_IIC was developed. The wedge splitting tests and mode II fracture tests for non-notched specimens were carried out simultaneously for concrete specimens which were prepared with aggregates in different volume fractions of 19vol%, 25vol%, 31vol% and 37vol%. The meso-scale simulations of corresponding mode II fracture tests were also performed by the cohesive zone model combined with the dot matrix method. The results show that as the aggregate volume fraction increases, the mode I fracture toughness K_IC, mode II fracture toughness K_IIC and the values of K_IIC/K_IC increase significantly. It was further found that the relationship between the values of K_IIC/K_IC and aggregate volume fraction can be described by a logarithmic function. The trapping-bridging of aggregate plays a leading role in the toughening of concrete, and its toughening effect is much greater than that caused by crack deflection or crack shielding. The meso-mechanical relation between K_IIC and aggregate volume fraction was developed, and its prediction agrees well with the experimental data. The mechanical response and fracture morphology from meso-scale simulation of mode II fracture of concrete are in good agreement with those of experimental tests. Furthermore, the scalar stiffness degradation (S_DEG) development of the cohesive element can be utilized to characterize the damage evolution of concrete at meso-level, which is valuable to the prediction of macro performance.