Wireless body sensor networks (WBSN) provide health related information for monitoring or professional analysis by collecting various signals of human body or environment information with sensors. But different data acquired in many applications have different transmission requirements. The dropping of life-critical messages could possibly create life threatening results if the network is not reliable. To improve the reliability this paper proposes a novel reliable medium access mechanism (RMAM) which guarantees transmission of data with different priorities in less delay and energy consumption. The mechanism is designed and evaluated by Castalia. The improved performances of latency, packets breakdown and energy consumption are analyzed and depicted with comparison.