Based on impedance characteristics of equivalent power-frequency component variation under different situation of commutation failures in HVDC system， combined with directional protection principle for power-frequency component variation， the influence mechanism of communication failure on the operation of directional relays for the different faulted transmission lines is analyzed. In addition， conditions of refusal to act of directional protection of faulted line under various situations are also deeply analyzed. The correctness of fore-mentioned analysis is confirmed with PSCAD/EMTDC simulation. 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）.