A nonlinear mechanical model for sub-micron cantilevers was analyzed in detail and its physical mechanism was researched to provide a theoretical basis for experiments.The electrostatical force was used to achieve the resonance state cantilever beams and a Polytec laser Doppler vibration measurement system was taken to observe the frequency response curve. Experimental results show that the beams have significant nonlinear effects (the spring softening effect) and the nonlinearity is relatively independent on the AC voltage, but it is markedly enhanced with increasing the DC voltage. The obtained maximum peak shift is 0.5 MHz and extracted first-order mechanical elasticity coefficients are 79.62, 31.75, and 14.92 N/m, respectively. Furthermore,the deviation of the experiment was also disccussed and analyzed.The effects of overetching by wet chemical etching on the stiffness and frequency response were stimulated by ANSYS software.In conclusion, the corresponding simulation results are well coincident with the experimental data.