Vehicular Ad Hoc Networks (VANETs) employ broadcasting way to support safetyrelated services, which have strict performance requirements, such as high reliability, low delay and scalability, etc. However, due to ever-changing vehicle density and the stringent requirements of safety applications, efficient and effective broadcasting of safety messages faces many challenges. In this paper, we propose a joint transmission Power and Contention Window (CW) size Adaptive Control (PCAC) scheme to improve the performance of safety-related services in VANETs. The transmission power dynamically adapts according to the local vehicle density estimation to solve conflict between transmission range and interference. The zero CW and minislot combined with based-receiver implicit acknowledgement mechanism are used, and the preemptive priority and reliability transmission of emergency messages are thus ensured. Coarse adjustment combined with fine adjustment approach is adopted to tune CW size of basic safety messages, which is based on the collision rate estimated from packets received from neighbor nodes, and thus the system throughput is improved. The PCAC scheme works in a distributed way and without extra communication overhead. Extensive simulation results demonstrate that the proposed PCAC scheme can significantly improve the performance of the safety-related services in terms of delay, packet delivery ratio and throughput.