As elementary components of smart materials, ferroelectrics, and their mechanical behaviours, have recently attracted scientists' attention. The nonlinear constitutive behaviours of ferroelectrics should be considered by the designer. In this work, under the framework of the internal-variable theory, the evolution of mesostructures (the patterns of internal domain rearrangement) and the incremental nonlinear constitutive relations are formulated. The simulation and experimental results are compared and discussed. The conclusions can be used for reliability evaluation and component design.