Abstract: Space Time Block Coding (STBC) technique could achieve transmitting diversity gain and be decoded easily, and Orthogonal Frequency Division Multiplexing (OFDM) is robust to multi-path effect in the underwater acoustic (UWA) channel and could reach higher spectrum efficiency. A novel high speed UWA communication system based on STBC-OFDM technique is proposed. It was tested in a lake. A bit rate of 3.328kbps over a bandwidth of 4.32kHz was approached within 1200m with the bit error rate (BER) lower than 10-5 in the lake. These results suggest that STBC-OFDM technique is an appealing solution for high data rate underwater acoustic communications.