With the success in synthesizing large-area polycrystalline heterostructure with seamlessly stitched graphene and hexagonal boron nitride (h-BN),understanding the mechanical properties of grain boundaries (GBs) in these heterostructure is of both scientific and technological importance.A recent work demonstrated that GBs in hybrid 2D materials tend to be non-bisector and obey a universal law to optimally match the heterogeneous grains.Combing molecular dynamics simulations and disclination theory,we study the mechanical properties of heterostructure with optimally matched GBs.we show that the detailed arrangements of defects in bisector and non-bisector GBs are important to the mechanical properties.Bisector GBs in hybrid nanosheets obeys the similar trend as GBs in pure graphene,and the GB strength can either increase or decrease with the tilt.However,heterogeneous grain with optimally matched GBs show contrasting trend that more GB defects could counter intuitively give rise to higher strength in tilt GBs.We find that such contrasting dependence of bisector and non-bisector GB normal strength on grain misorientation stems from the inclination of pentagon-heptagon defects under pulling perpendicular the GB.The investigation gives helpful insight into the strength characteristics of hybrid two-dimensional nanomaterials based electronic and optical devices.