Media Access Control (MAC) protocols in Wireless Networked Control System (WNCS) must minimize the radio energy costs in sensor nodes. Latency and throughput are also important design features for MAC protocols in the applications. But most of them cannot guarantee quality for real-time traffic. This paper studies the state of the art of current real-time MAC protocols, and then introduces a medium access control protocol and a improved protocol that provide multiple priority levels and hard real-time transmission. The channel is accessed by sensors according to their priorities. Sensors send frames in a round manner with same priority. The fairness between different priorities is provided. The channel access procedure is divided into two stages: broadcast period and transmission period. Simulation and experiment results indicate that our protocol provides high channel utilization and bounded delays for real-time communication and can be well applied in the many fields especially the dynamic wireless sensor networks.