Because the coding matrix in the orthogonal space time block coding (OSTBC) has the orthogonal properties,the correlation matrix of the virtual channel matrix is a diagonal matrix.A characteristic parameter,which is the ratio of the energy of non diagonal elements of primary of the correlation matrix to the energy of its primary diagonal elements, is proposed.A new method for blind recognition of OSTBC is proposed. MDL criterion is used to estimate the dimension of the signal subspace, that is,the number of symbols containing in each group of transmitted signals.The simulation results demonstrate that the proposed method achieves good performance to identify OSTBC when the numbers of transmitting antennas and receiving antennas are different.The estimation performance trend of the number of symbols is the same as the probability of correct identification.