AbstractSeveral size-effect laws (SELs) have been proposed and verified for concrete-like quasi-brittle materials. However, there is no SEL that can quantitatively describe the influence of related factors on size effect in carbon fiber–reinforced polymer (CFRP)-strengthened lightweight aggregate concrete (LWC) components without stirrups, for example, the shear failure of beams. The main purpose of the present study is to develop a novel SEL for the evaluation of shear strength of CFRP-strengthened LWC and normal-weight concrete (NWC) beams. A 3D mesoscale simulation method is employed to explore the structural behavior of both LWC and NWC beams wrapped with CFRP sheets, in which both the concrete heterogeneities and the steel/concrete interactions are considered. Based on the modeling approach, the influences of shear span ratio and CFRP ratio on the shear failure behavior and the size effect on shear strength are studied. Also, the shear contribution by CFRP on the LWC beams having different shear span ratios under different structural sizes (i.e., beam-depth) is examined. Finally, a novel size effect law that can quantitatively describe the influence of shear span ratio and CFRP ratio on shear strength of CFRP-strengthened LWC beam is developed, and it is verified with the available test data and simulation results.