In this article the dynamic behavior of clamped-clamped micromachined mechanical resonators working at a frequency of 150 kHz was characterized by electrostatic excitation and detection via both capacitive coupling and optical methods. The experimental results by optical measurement show that this system behaved very much like a simple harmonic oscillator at low actuation levels while significant nonlinearity was observed at higher actuation levels. The nonlinearity identified as "spring softening" type occurred when the product of the dc bias voltage and the ac driving voltage exceeded 0.04 V-2. Dependence of resonance frequency on dc bias voltage was investigated and the experimental results are in agreement with theoretical predictions. This work also demonstrated that optical detection is much less susceptible to parasitic interference than motion detection via capacitive coupling. Therefore, optical techniques will yield better characterization of the mechanical properties of the resonators. (c) 2006 American Vacuum Society.