Resonator fiber optical gyroscope (R-FOG) uses ring resonator to enhance the Sagnac effect, the detecting methods have open-loop and closed-loop, the phase modulator′s controlling signal has analogous and digital in circuit. Compared with analogous modulation, the method of digital closed-loop detection has a big dynamic range and high sensitivity. In resonator fiber optic gyroscopes by digital modulation, imperfect serrodyne′s reset amplitude V2π has influence on the rotation sensing errors and the scale factor non-linearity. The effects of imperfect V2π on the system are analyzed theoretically. If the amplitude is not exactly V2π, two electric fields, Ecross and Ebar cannot result in optimum destructive interference, thus the resulting waveform has errors. Based on the sum of electric fields expression in detector, numerical calculations of rotation sensing errors associated with imperfect serrodyne amplitude are given for resonator fiber optical gyro using two frequencies modulation. At last, some solutions for substantially reducing errors are presented.