We present a new method for estimating the thermal noise at off-resonant frequencies in a mechanical system. In this method, the mechanical conductance is directly measured at anti-resonant frequencies, and then the fluctuation-dissipation theorem is applied to the measured conductance in order to evaluate the thermal noise. To confirm the validity of this method, we demonstrated a direct measurement of the mechanical conductance of a 2-mode oscillator at an anti-resonant frequency. The measured conductance was in good agreement with the value calculated by the normal-mode expansion. (C) 2000 Elsevier Science B.V.