Most existing techniques for space time block coding (STBC) systems have assumed full decorrelated radio channels, which may not be the case in practice. In this paper a subspace based approach of semi-blind channel estimation is proposed for precoded STBC systems, in which multiple-input multiple-output (MIMO) channels are partially correlated. By carefully designing the transceiver of the precoded STBC systems, correlated MIMO channels (where correlation is due to antenna geometry) can be converted to full independent MIMO channels. Then a conventional subspace based channel estimation method is applied to such precoded STBC systems. Simulation results demonstrate the performance of the proposed algorithm.