The accurate measurement of water content in the turbine oil has great significant for guiding steam turbine safety and economy running. A method of the on-line measurement of the water content in the turbine oil based on microwave resonant cavity perturbation is presented in this paper, The author deduced the formula of the volume proportion of water content in turbine oil, the resonant cavity and the measuring system of this method is simple. The precision of measurement can be improved by selecting work pattern with high quality factor, advancing the measurement precision of the relative frequency deflection and adopting the lower thermal expansion coefficient material and the especial resonant cavity configuration. Ten PPM water content in turbine oil can be distinguished by this method.Copyright © 2005 by ASME