Simple and accurate prediction methods of gear unit noise have been desired. This paper offers a new prediction equation for spur and helical gears under speed reduction service. A semi-empirical equation was developed by means of the addition of a dynamics term to Kato’s equation which represented the overall noise level by using gear’s specification data. As the proposed method takes into account the gear error characteristics in the vibration analysis of gear pairs, it can calculate the noise levels considering the influence of the tooth flank finishing method. It is shown that the predicted values agree with the measured values in an experiment within a range of approximately 5dB, under almost all the operating conditions and for all test gears having different finishing methods such as hobbing, Niles-type grinding and Maag-type grinding. Moreover, good agreement with data of some actual gear units indicates that the developed method may be used for general application.