Current interest in two-dimensional materials extends from graphene to others systems such as single-layer hexagonal boron-nitride (h-BN), for the possibility of making heterogeneous structures. Here, we report mechanical properties of h-BN and its band structures tuned by straining by using the density functional theory calculations. Young’s modulus and bending rigidity for h-BN are isotropic; its failure strength and failure strain show strong anisotropy. A small fraction of antisite defects in h-BN can largely decrease its mechanical properties. We reveal that strain can tune single-layer h-BN from an insulator to a semiconductor.