This paper presents the development of nonlinear coupled electro-thermo-elastic constitutive relations applicable for piezo materials. These nonlinear constitutive relations model the experimentally observed hysteresis and butterfly loops accurately and, can be easily incorporated in structural analysis of smart structures. Using this nonlinear formulation, static analysis of smart composite plates has been carried out to bring out the effect of polarization-electric field nonlinearity on the deformation of composite plates. Results of static analysis are correlated with the experimental data and it is shown that nonlinear formulation provides a better correlation with the experimental results than the linear case. © 2010 Springer Science+Business Media B.V.