Abstract: The noise due to the birefringence modulation of a nonideal phase modulator in a fiber-optic gyroscope is investigated. It is found that this noise contributes largely to long term bias drift of the system, and that the phase of this noise has a definite π/2 relation with that of the rotation signal. Theoretical analysis based on the Jones Matrix representation coincides quite well with the experimental results. Measures to reduce the birefringence modulation noise are discussed.