BaTiO₃ single crystal with superior dielectric, pyroelectric, piezoelectric and electro–optic properties has triggered much attention due to numerous potential applications in microelectronic devices, sensors and micro/nanoelectromechanical systems. However, one of the main disadvantages for BaTiO₃ single crystal is that after long–term working under mechanical field or electric field, cracks may be nucleated and propagated. The crack nucleation and propagation in ferroelectric materials are closely related to the domain configurations, and their relationship with crack propagation, nucleation and domain switching, is not known clearly, so it is imperative to take more effort to research this relationship. Here, the relationship of domain switching with crack propagating quickly as well as sub–critical crack growth in ambient air for constant deflection sample of BaTiO₃ single crystal has been in situ tudied using a polarized light microscopy. The results indicate that domain switching occurs first and he volume fraction of swiched bnds increases dring loading. When the load reaches to a critical value, crack will be initiaed nd propagated qickly. Keeping the constant deflection, the domain switch could still occur but very slowly, i.e., adsorption of water in air is able to promote domain switchng. As a result of adsorption–enhanced domain switching, cracks are propagated lowly n air, that s say, sub–critical crack growth or stress corrosion cracking in air could occur for BaTiO₃ single crystal under constant deflection.