A lot of experiments have proved that the basic reason of the nonlinear and hysteresis constitutive curve in ferroelectric ceramics is domain switching. The microscopic domain switching behaviors of BaTiO
_3
single crystals have been studied detailed. The forward and sideway motion of 180° domain wall has been measured by direct stroboscopic observation. The velocity expressions are given. In order to get the evolution equation of the volume fraction of domain switching, a square domain switching model is developed according to the relationship between the domain wall velocity and applied electric field. The model is compared to available experimental data of BaTiO
_3
single crystals. It turns out that the model can represent the typical nonlinear electromechanical coupling phenomena. The evolution equation of the volume fraction of domain switching is reasonably consistent with a previous phenomenological model. Furthermore the evolution equation based on microscopic domain motion can further improve the microscopic phenomenological model.
