The advantage of the polarimetric fiber laser current sensors over other types of current sensors is direct readout of electrical current, measured as a difference of two beat-frequencies. In this paper, frequency null-drift caused by the deviation of the polarization rotation angle in the Faraday rotation mirror (FRM) from 90° is analyzed, and that it is the main source of unstability to variation of temperature is emphasized. Experimental results of this type of current sensors confirm the theoretical predicts and indicate that the suggested method to reduce the frequency null-drift is effective.