In this letter, a fast energy-based method of antenna efficiency measurement in a reverberation chamber (RC) in time domain (TD) is proposed. The antenna efficiency is determined by comparing the received energy from the antenna under test with that from the reference antenna. The measurement results are compared with that from the conventional reference antenna method in the frequency domain (FD). The proposed method needs no mechanical stirring or only a few mechanical stirrer positions, which could reduce the measurement time significantly.