  Ultrasensitive detections have been proposed as an application of
optomechanical systems. Here we develop an approach to mass sensing by
comparing the detected quadratures of light field coupled to a mechanical
resonator, whose slight change of the mass should be precisely measured. The
change in the mass of the mechanical resonator will cause the detectable
difference in the evolved quadrature of the light field, to which the
mechanical oscillator is coupled. It is shown that the ultra-small change
$\Delta m$ from a mass $m$ can be detected up to the ratio $\Delta m/m\sim
10^{-8}-10^{-7}$ by choosing the feasible system parameters.
