The characteristics of nonlinear harmonic generation of bulk acoustic wave (BAW) modes in surface acoustic wave (SAW) devices were studied and presented. Specifically the BAW modes were nonlinearly generated (f-2f-3f) by the fundamental SAW mode in a 128° YX LiNO3 temperature compensated SAW (TC-SAW) resonator. These BAW modes interfered with the desired SAW mode of the TC-SAW resonator. The method and formulation of nonlinear frequency response of piezoelectric BAW and SAW was presented, and FEM was used to solve three sets of coupled nonlinear equations in the frequency domain. For SAW devices, nonlinearly excited BAW waves in 128° YX LiNO3 TC-SAW devices had been observed experimentally and reported . We demonstrated our formulation/method for this nonlinear harmonic generation problem, and we obtained good, consistent simulation results. The 2nd harmonic (H2) generation was due to a nonlinearly excited bulk wave mode polarized and travelling along the thickness direction of substrate. The mode was almost normal to the top and bottom surfaces of substrate, forming a standing wave between top and bottom surfaces. Therefore, the substrate thickness, and bottom surface roughness had effects on the H2 response. The H3 response was due to a similar type of nonlinearly excited BAW wave.