We propose a subspace-based blind channel estimation method for space-time coded OFDM system. Using only the redundancy induced by OFDM modulation and space-time block coding (STBC), channel state information (CSI) can be blindly estimated up to two scalar ambiguities, even with single receiving antenna. Compared with other blind channel estimation method for space time OFDM systems, this method needs neither pre-coding nor over-sampling, and thus has higher system data rate and lower complexity.