Abstract: A double differential cooperative transmission (DDCT) scheme using quadrature amplitude modulation (QAM) is proposed to overcome carrier frequency offset in cooperative transmission. A closed-form symbol error rate (SER) formulation, an SER upper bound, and an asymptotic SER approximation at high SNR are derived. Based on the asymptotic SER approximation, an approximate solution of optimum power allocation (OPA) at high signal-to-noise ratio (SNR) is derived. The simulation results demonstrate that the approximate solution of OPA also provides a good solution to a realistic moderate SNR scenario. The SER performance of DDCT with QAM modulation and DDCT with phase shift keying (PSK) modulation is compared in condition of same modulation order. The comparison results demonstrate that DDCT-QAM outperforms the DDCT-PSK, when the modulation order is larger than 8. Specially, when the modulation order equals to 64, DDCT-QAM obtains a 4.5 dB SNR gain.