Ferroelectrics and their mechanical behaviors have recently attracted attention. An attempt has been made in this investigation to establish macroscopic constitutive relations of ferroelectric single crystals in terms of micromechanics approaches. On the basis of analyzing the coupled microscopic electroelastic fields and using the internal variable theory, the domain switching conditions, the evolution of microstructures, the incremental mechanical and electrical relations are formulated. The model is then used to approximately simulate the nonlinear behavior of polycrystalline ferroelectrics. The simulation gives a fairly good approximation for both ferroelectric single crystal and ceramics. The method proposed can be applied to describe both the mechanical and electrical behaviors of electronic materials. The final conclusions may be used in the design and reliability evaluation of ferroelectric components.