Abstract: A subspace-based blind channel estimator has been developed for space time coding orthogonal frequency division multiplexing(STC-OFDM) system with two transmit antennas and two receive antennas. A condition that guarantees multichannel to be identifiable to two ambiguity parameters is given. The method does not need precoding or constant-modulus(CM) modulating. Finally, simulation results demonstrate the performance of the proposed blind algorithm.