2D materials always suffer from ultralow toughness due to the lack of intrinsic toughening mechanism. Based on in situ tensile test, we show that single-crystal monolayer h-BN is the toughest 2D material measured to date. The effective energy release rate (~160 J/m2) is one-order of magnitude higher than its Griffith energy release rate and that of graphene, while their strength is very close. Stable crack propagation in monolayer 2D h-BN is observed and the corresponding crack resistance curve is obtained for the first time in 2D crystals. Inspired by the asymmetric lattice structure of h-BN, an intrinsic toughening mechanism without loss of high strength is validated based on theoretical efforts. The crack deflection and branching occur repeatedly due to asymmetric edge elastic properties at the crack tip and edge swapping during crack propagation, which toughens h-BN tremendously and enables stable crack propagation not seen in graphene.