A novel time-frequency cooperative communication strategy is proposed for coded OFDMA system over quasi-static frequency-selective Rayleigh fading. To take advantage of the frequency dispersion, each user transmits his partner's and his own data on different subcarriers spreading over the entire available bandwidth. The allocation of subcarriers to each user and his partners can be either pre-specified or dynamically determined. Compared to other cooperative strategies using the time division orthogonal channel allocation, this cooperative strategy benefits from the freedom of subcarrier allocation and the "created" block fading channel during the cooperation process. The performance characteristic is developed in terms of pairwise error probability (PEP) and frame error probability to illustrate the improved diversity gain for various cooperation scenarios. Computer simulation results are provided to verify our theoretical analysis.