The performance of RFID MAC protocols greatly influences RFID systems. Previous research work concentrates on reducing either the recognition time or the energy consumption of tags. In this work, we develop an efficient RFID MAC protocol which shows good performance in terms of both low energy consumption at tags and average recognition time to recognize one tag. We propose an efficient M-ary detecting tree RFID MAC protocol which uses a specially designed bit-detecting mechanism to recognize tags. Simulation results proves that the proposed protocol outperforms existing schemes in the literature in both energy consumption at the tag side and average recognition time.