MEMS resonators have been widely studied due to their advantages compared with those made of quartz crystals in sensing, timing, and filtering applications. For example, doubly clamped beam micro resonators are widely used in resonant sensors and oscillators due to their structural simplicity. As the beam stretches, nonlinear behavior is readily observed at large vibrating amplitudes. The resonance frequency of a nonlinear micromechanical resonator has a dependence on its modal amplitude known as A-f effect, which is used for frequency tuning through changing the modal amplitude. The other vibration modes, however, can also be triggered as the amplitude of main mode reaches a certain level and accordingly strong nonlinear behavior like amplitude saturation can be observed. Traditionally, the nonlinear behavior of micromechanical resonators is purposely avoided in the design or cancelled in the real applications. However, the research shows that the nonlinear behavior can also be employed to realize certain functions for improving the performance of micro devices for frequency reference and timing purpose. In this talk, I will introduce our recent work of exploring nonlinear phenomena in MEMS resonators and developing high performance sensors based on these findings.