For high voltage direct current（HVDC） transmission lines， the transient fault characteristics are that， if the fault point is in the line， the currents measured at both convert stations will be increased， and with the same changing trends; if the fault point is out of the line， the current measured at one convert station is increased， and the current measured at the other convert station is decreased， and with the opposite changing trends. According to such characteristics， and using the currents measured at both convert stations， a new pilot protection principle for HVDC transmission lines based on current fault component is proposed. This principle has no limitation on the sampling frequency. Internal and external faults of the line can be effectively identified only using the current fault component at both convert stations. On the other hand， the setting principle of the current fault component is given. The simulation results show that the method proposed can identify the fault reliably and rapidly in different conditions.