<p>Stress concentration and nonlinear buckling created by grain boundaries in two-dimensional (2D) structures, along with topological effects and various geometric defects, are influential factors in determining the mechanical properties and fracture behavior of 2D materials. This study investigates the mechanical and fracture behavior of graphene and boron nitride (BN) nanosheets in relation to temperature and types of grain boundaries. The mechanical performance of graphene and BN nanosheets, both with and without grain boundaries, as well as graphene-BN hybrid structures, was assessed by focusing on the types of grain boundaries as the primary sources of stress concentration. Molecular dynamics simulations was employed as a cost-effective technique to model and test 2D sheets, utilizing appropriate potential functions and boundary conditions. The results revealed that increasing the temperature from 100 to 900 K in the armchair and zigzag directions decreased the mechanical properties of both graphene and BN structures. Notably, BN exhibited a greater decrease in mechanical properties than graphene, and the presence of grain boundaries further diminished the mechanical properties of both materials. The lowest values of Young's modulus, failure stress, and failure strain for BN-7–5–2 at 900 K were found to be 548.27 GPa, 42.82 GPa, and 0.099%, respectively. Additionally, grain boundaries and temperature variations played a critical role in the fracture process of the graphene-BN hybrid structure. These findings can be extended to more complex cases to provide a deeper understanding of the next generation of 2D structures.</p>