A mechanical resonator based on torsional resonance has been fabricated in our facilities to sense infrared radiation. Actuation and detection are both electrostatic. Reaching phonon noise is highly desirable in order to get low noise uncooled infrared detectors. Therefore a high dynamic range has to be reached to increase as much as possible the overall signal to noise ratio and minimize the contribution of amplitudes noises (thermomechanical or electrical) to frequency noise. The dimensions of resonator body are similar to most clamped-clamped NEMS and our devices are thus also greatly affected by nonlinearities. We present here a nonlinear model for mechanical behavior around fundamental mode resonance taking account of both mechanical and electrostatic nonlinearities. The model correctly reproduces the nonlinear behavior observed for different resonator designs. We experimentally observe on some devices a compensation of hardening effects, allowing a linear operation of torsion angle up to 13°. The model provided in this work allows an engineering strategy in order to design high linear dynamic range for fundamental torsional mode. © 2017 Elsevier B.V.