A vehicle collision avoidance system is implemented in a host vehicle. A wireless communication module in the host vehicles wirelessly broadcasts vehicle information packages of the host vehicle and receives external vehicle information packages from other neighboring vehicles. Based on the received vehicle information packages, a collision avoidance process is performed. The process has steps of mapping coordinates system, categorizing collision zones, determining whether a possible collision position exists, calculating a collision time and outputting warning messages. The estimations of the possible collision position and the collision time are not affected by the positions of the neighboring vehicles. Therefore, the neighboring vehicles approaching the host vehicle from different direction are effectively monitored.