The WAVE (IEEE 1609) standard and vehicular technology creates a new opportunity for telematics service. Safety is one of the top issues of telematics service. In this paper, we focus on the safety application and WAVE communication to provide proactive safety which can alert drivers before the accident occurs. An algorithm named Collision Pre-Warning Algorithm (CPWA) was proposed to tackle the aforementioned issues. In CPWA, vehicles broadcast Relationship Information (RI) to neighboring vehicles periodically. Each vehicle calculates whether potential collision may happen or not upon the RI messages that were received. After calculation, the CPWA sends warning messages to drivers if the collision will occur before several seconds. The results show that CPWA has good performance and can always alert drivers suitably.