Abstract: The error performance of vehicle-relayed inter-vehicular communication (IVC) systems is studied based on double-Rayleigh fading model. The statistical properties of the end-to-end signal-to-noise ratio (SNR) are analyzed and the solution to optimal power allocation (OPA) coefficient is presented for the system. A generalized expression of the lower bound on average symbol-error-rate (ASER) for various M-ary modulations is derived through moment-generating-function based approach for the cooperative IVC systems. The method to finding the solution of Chernoff union bound on error probability of the system is provided. Simulation results show that the ASER decreases by applying vehicle-relaying technique in vehicle-to-vehicle communications. The differences between the exact ASER value and its lower bound increase slowly with SNR and the range to the ASER is dete rmined by its lower bound and Chernoff upper bound. Numerical results further indicate that the transmit power can save 3 dB at most through OPA than equal power allocation method in the cooperative IVC system.