Nonlinear resonance characteristics of transverse type-PZT ceramic resonator were investigated, and their nonlinear coefficients were calculated using the nonlinear theory proposed by Duffing. Resonance characteristics of sample showed nonlinearity by the thermal effects due to driving current. Nonlinear coefficients greatly affected by sample dimension, however comparing with respect to current density, it was almost constant. Nonlinear coefficients were not changed as driving current increased upto 40 mA/$ extrm{cm}^2$, when $alpha$ and $eta$ was 920 and -10.6, respectively, while nonlinear coefficients exponentially increased beyond the current density of 40 mA/$ extrm{cm}^2$. Nonlinear coefficients were slightly increased as temperature increased.