In future wireless communication systems such as 3rd generation mobile systems, increasing the downlink capacity from the base stations to remote units becomes more important due to high data rate applications such as wireless internet access. Turbo Codes and Space-Time Transmit Diversity (STTD) are two of the most recently techniques to improve system performance and capacity. In this paper, the performance of turbo codes with transmit diversity is studied. The effect of number of transmits antennas, puncturing, and frame length are investigated over Rayleigh fading channels. The simulation results show that a significant gain is achieved when using turbo codes with space-time transmit diversity technique.