A novel scheme for space-time block coding with a variable transmit diversity gain is proposed for OFDM systems in the paper. The OFDM symbols are divided into two groups of orthogonal sub-symbols with each group coded in Alamouli STBC in the coding, and the new scheme for STBC is constructed by differently combining the coded data of each group with the different transmit diversity gain to be achieved by linear processing and correspondingly combining the received data on the sub-carriers. The proposed scheme is feasible to be used in video transmission and a new unequal error protection applied to MPEG-4 video transmission is discussed. The results of computer simulation show that the proposed scheme has double diversity gain and quadruple diversity gain. The new unequal error protection scheme has a gain more than 8 dB and higher efficiency than the UEP based on the RCPC codes.