In general, gear tooth surfaces of automobile transmissions are modified to reduce gear noise. This paper presents an analytical procedure for optimum surface modification, which is designed to supersede the experimental procedure by trial-and-error. First, gear noise level is shown by experiment to correspond with the scale of transmission error of a gear pair. Second, a computer program to calculate the transmission error is described. All factors of tooth surface modification and load condition can be fed to the program.For the test gear pair, the specific values of each factors to minimize transmission error, with due consideration of the tolerance determined by the production capacity, were determined and as a result a significant improvement in gear noise level was achieved.This new procedure can be used effectively for the design of quiet gears.