Capacitor voltage transformer cannot transmit wide frequency band voltage fault travelling-waves effectively， so that traditional voltage fault travelling-wave based directional relay cannot be applied to power system protections. Then a polarized current travelling-wave based directional relay（PCTDR） for transmission lines is put forward. This directional relay judges the fault direction by comparing the initial polarity of fundamental frequency components in voltage fault transient travelling-waves with the wave front polarity of current fault initial travelling-wave. This method resolves the problems that the traditional travelling-wave directional relay cannot be utilized in power system because capacitor voltage transformer cannot transmit wide frequency band voltage travelling-waves effectively. The basic principle and protection algorithm of PCTDR are presented. Simulation model of a 750 kV transmission system has been constructed with alternative transient program（ATP）. Simulation results verify that PCTDR has good operation performance because it has fast operation speed and is immune to fault types， fault distance， fault path resistance and current transformer saturation.