Abstract: Lightweight aggregate concrete (LWAC) is more and more utilized in engineering structures because of the advantages of light weight and good thermal insulation performance. Both lightweight aggregate concrete and ordinary concrete were regarded as three-phase composites consisting of the aggregate particles, mortar matrix and interface transition zones (ITZs) between them. A 3D meso-scale mechanical model was established to simulate the shear failure behavior of concrete beams without web reinforcement. The shear failure modes and failure mechanism of ordinary and light aggregate concrete cantilever beams with different structural sizes under monotonic loading were modelled and studied. The size effect on the nominal shear strength was examined. In addition, the accuracy and safety of the formula for calculating the shear capacity of the relevant design codes are preliminarily discussed based on the simulation results. The results show that different from the ordinary concrete beam, the LWAC specimens sustained serious damage due to the lower strength of the aggregates. The shear failure modes of concrete beams with different sizes were basically the same. An obvious size effect on the nominal shear strength of both LWAC and ordinary concrete specimens was observed. Compared with the ordinary concrete beams, lightweight aggregate concrete beams showed a stronger size effect in shear failure behavior.