The aim of this research article was to investigate the effect of grain boundaries (GBs), and hydrogen functionalisation on the fracture toughness of bi-crystalline hexagonal boron nitride (h-BN) nanosheets. Molecular dynamics based simulations were performed in conjunction with the reactive force field to study the crack tip behaviour in single and bi-crystalline h-BN nanosheets. Atomistic simulations help in predicting a positive effect of the GB plane in the near vicinity of the crack tip. The density of 5|7 dislocation pairs significantly affects the fracture behaviour of bi-crystalline h-BN nanosheets. Additionally, the distance of the GB plane from the crack tip, and limited hydrogen functionalisation of GB atoms, further help in improving the fracture toughness of bi-crystalline h-BN nanosheets. Hydrogen functionalisation helps in inducing out of plane displacement at the GB plane, which helps in arresting or retarding the crack propagation. It can be concluded from the results that instead of deteriorating, geometrical defects such as GBs can also be used to tailor the fracture toughness of h-BN nanosheets. This study on the fracture toughness of bi-crystalline h-BN nanosheets helps in complementing the research on using porous h-BN nanosheets as nanomembranes for water desalination and ion separation.