A new countermeasure to current transformer（TA） saturation， namely， current polarity comparison method， for differential current protection of transmission line is proposed. It compares the polarity of the saturated current of TA and that of the differential current during internal faults and external faults with a special analysis method. EMTP simulation results show that the proposed method is able to successfully block the differential current protection during external faults even when TA saturates， and also quickly unblock it during internal faults， especially when an evolving fault occurs.