The nonlinear error of scale factor of depolarized fiber gyroscope(D-FOG)with closed-loop has been investigated theoretically and experimentally for the first time.Our results show that the nonlinear error of depolarized FOG with standard single-mode fiber coil,which is larger than that of polarization-maintaining FOG,is caused by the different modulated depths of serrodyne for the two states of polarization.Practical methods to reduce nonlinear error of depolarized FOG are suggested in this paper.