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A subspace based semi-blind channel estimation method is proposed for space time coding orthogonal frequency division multiplexing (STC-OFDM) systems with two transmit antennas and two receive antennas, which have no cyclic prefix. The algorithm of blind estimation is first given, and a few pilots are used to solve the disadvantage of high computation and slow convergence. By theoretical analysis, it is established that multi-channel identifiability is guaranteed up to two scalars. The method does not need preceding or constant-modulus modulating and is not limited by the real inputs or symmetric inputs, simulation results demonstrate the performance of the semi-blind algorithm.