The through currents in transmission lines with internal faults are defined. The studies on through current show that internal faults of transmission lines with heavy-loads have an effect on the sensitivity of the traditional current differential protection under condition of steady currents， and the different fault location has different abilities of detecting high resistance internal faults. By improving the restraining characteristics， an improved steady current differential protection criterion is proposed. The comparison between the improved criterion and traditional one shows the same security on external faults， and the improved criterion has higher sensitivity and capability of detection on the faults with higher fault resistance. The simulation results from the faults with the two protection criteria in a 750 kV line verify that the proposed one has achieved a great improvement on sensitivity of internal faults and the abilities of detecting high resistance internal faults. It is an ideal current differential protection criterion for high voltage and ultra high voltage（UHV） transmission lines.