In this paper, we report for the first time the detailed analysis of the nonlinear behavior of a Lame-mode SOI bulk resonator. The measured resonant frequency of the resonator was 6.35 MHz with a quality factor of 1.7 million in the ambient pressure of 0.02 Pa. We used the two-step semi-analytic approach to characterize the model parameters of the resonator and the nonlinear model of the resonator was verified by the experimental results. Our study shows that the Lame-mode bulk resonator has three orders of magnitude larger maximum energy storage capability than the flexural beam resonator, leading to improved overall phase noise performance of the resonator-based oscillator.