Piezoelectric transducer (PZT) are often used for providing reciprocal sine phase modulation in all-fiber optic gyroscope (FOG) and its non-linearities can affect the accuracy of FOG. In this article, expressions of the first, second and fourth harmonic components of the FOG signal are derived theoretically providing that the second harmonic phase modulation exists in open loop FOG. Influences of phase modulator non-linearities on measurement of work point and signal demodulation are analyzed theoretically and tested by experiments. Both experimental results and theoretic analyses indicate that steps must be taken to decrease the amplitude of non-linearities in phase modulation.