It is important to monitor the moisture content of the oil during the operation of turbine. Such applications require the sensor can provide relative long time, continuous, and accurate measurements. In this paper, a new integrated on-line microwave moisture sensor system is presented. The moisture sensor is based on the principle that the permittivity of the two materials mixture can be calculated by measuring the resonant frequency. The relationship between the moisture content and resonant frequency is obtained by using of the perturbation theory analysis and dielectric mixture theory. Detailed sensor design is on the basis of the consideration of sensitivity, dynamic range, liquidity, coupling and temperature calibration. The measure system is implemented with circulator, PLL, detector, temperature sensor, signal processing part and LCD display. Temperature calibration in the practical application is also included.