Abstract: A fiber-optic gyro is a kind of optical fiber sensor for the angular velocity. In this paper, the fundamental principle of fiber-optic gyro based on Sagnac effect is reviewed, and the integrated optical phase modulator, which is a key device of the gyro and is our main task in the work, are analysed. According to the yielded equations, the drive voltage and frequency bandwidth in modulation of fiber-optic gyro are calculated, and the calculation results can be contributed to a design of the modulator used in fiber-optic gyro.