The fact that the Wigner-Ville Distribution (WVD) can represent the energy of a gear vibration signal was used for the gear damage detection. A new gear fault detection parameter called NP4 is derived from the joint timefrequency Wigner-Ville Distribution. The novelty of the NP4 parameter is in application of the previously de® ned statistical parameter called kurtosis to the WVD data and its interpretation for gear fault detection. The important distinct feature of the fault detection parameter NP4 from other fault detection parameters, such as® gures of merit, is that it does not compare a measured gear vibration signal with the ideal one. Thus, the parameter NP4 can work for the fault detection without a long recorded vibration history of the gear. New techniques for enhancing the information content of the WVD of the gear vibration signal and the reliability of the parameter NP4 are also described. The utility of the gear fault detection parameter has been demonstrated using numerous gear vibration experiments. A correlation between the level of the gear tooth damage and the value of the gear fault detection parameter NP4 is demonstrated. The gear fault detection strategy based on the developed parameter NP4 is presented and investigated in the paper.