Based on the characteristics of DC-system impedance of power-frequency component variation under different conditions of commutation failures in HVDC system， the influence of the transient power converse caused by commutation failure on the operation of directional relays for the different unfaulted transmission lines is analyzed. The transient power converse may result in the wrong tripping of directional pilot protection of the unfaulted transmission lines. The response of directional pilot protection under different conditions of commutation failures are evaluated on the basis of the protection tripping logic. Simulations based on GIGRE benchmark HVDC system prove the correctness of the conclusion. The work is supported by National Mega-projects of Science Research for the 11th Five-year Plan（No. 2006BAA02A30） and National Natural Science Foundation of China（No. 50337010）.