Abstract: In this paper, a modulation classification algorithm based on fourth-order lag moment is proposed for STBC-OFDM（Space-Time Block Codes-Orthogonal Frequency-Division Multiplexing）signals in the single receiver antenna communication systems. Firstly, the theoretical values and variance of various signal constellations are obtained by computing the ensemble averages over the ideal condition. Then, the thresholds are gained by theoretical value. Finally, the automatic classification of modulation scheme is realized by comparing the experimental values and the thresholds. Furthermore, unlike other methods, this algorithm does not require any prior information of the type of STBC, and is well-suited for non-cooperative context. The simulation shows that the proposed algorithm performs well even at a low signal to noise ratio (SNR).