Abstract: A new way for detecting gear fault by signal processing is proposed, which is called TFF (Transient Frequency Fluctuation) analysis. The technique is based on the Time Domain Average and Hilbert Transform. Much analysis is made oa the modulation of gear vibration signal due to defects. Through FM demodulation, the frequency modulation signal TFF (t) can be extracted which is consistent in relation to gear defects. Experimental results show that the proposed approach can give earlier indication of gear malfunction than other conventional methods.