Transmission rate of every sensor node may is different in middle and high rate sensor networks. To support quality of service (QoS) requirements for different transmission rate, energy efficiency is a very critical issue to prolong the networks lifetime, an effective and fare MAC is necessary. A hybrid MAC protocol based on cross-layer optimization, called RHMAC (Rate-adaptive Hybrid Medium Access Control) for middle and high rate sensor networks is proposed in this paper. RHMAC combines the strengths of both CSMA and TDMA at the same time offsetting their weaknesses. Like CSMA, RHMAC achieves high channel utilization and low latency under low data rate. Like TDMA, achieves high channel utilization and fair under middle and high rate and reduces collision among two-hop neighbors at a low cost. RHMAC can switch between CSMA and TDMA depending on date rate automatically. Simulation result shows that RHMAC achieves much higher throughput and energy efficiency than other traditional hybrid MAC such as BMAC in multi-hop in middle and high rate sensor networks.