Abstract: Compared with those of concrete materials, the failure modes and mechanisms of reinforced concrete (RC) members are much more complicated. It is therefore hard to describe the size effect behavior of RC members by using the size effect theory of concrete materials. From the mesoscopic view, a three dimensional meso-scale numerical analysis model of RC cantilever beams was established to study the size effect of shear failure of RC cantilever beams. The feasibility of the simulation method was verified by the existing experimental data, and the influences of shear span ratio and stirrup ratio on the size effect of shear failure of RC cantilever beams was investigated. The simulation results indicate that:the shear span ratio has a great influence on the shear capacity of RC cantilever beams, but has little effect on the size effect; the increase of stirrup ratio increases the shear capacity of RC cantilever beams, and weakens the size effect of the shear strength of RC cantilever beams. Furthermore, combined with the influence mechanism of shear span ratio and stirrup ratio on shear strength of RC cantilever beams, a theoretical formula for the size effect of shear strength of RC cantilever beams was established based on the theory of the Size Effect Law (SEL) proposed by Bažant. The accuracy and rationality of the theoretical formula are verified by comparing the simulation results and the test data.