In order to solve the setting and coordination problems of the traditional back-up protection and the misoperation during flow transfer due to overload， a novel wide area back-up protection algorithm based on the distribution of fault voltage is proposed. This algorithm uses the value of fault voltage and fault current measured from one side of the line to estimate the fault voltage of the other side. The ratio of the estimated to the measured value constitutes the protection criterion. Unaffected by transition resistance， this algorithm will not misoperate when flow transfer due to overload occurs. The principle of this algorithm is simple， requires very little information， has no rigorous requirement on the synchronization of information from different sources and is convenient to implement. Simulation results show that the algorithm can successfully identify the faulted line whatever the type of faults.