Some papers on gear noise indicate that for helical gears,the vibrational excitation becomes a minimum when the gears in mesh rotate at a total tooth-contact ratio,where the variation of tooth-stiffnes may be minimized (1). Based on this finding, we have designed a pair of herlical gears, of which number of teeth in contact (2,3,or4) would be fixed at any instant of time during the gear rotation. Both the face- and total-contact ratios of involute tooth-face have often been considered to be integers, and a little higher value for the gear with inaccurate tooth-face.Meanwhile, it has been confirmed by computation and experiments that the vibrational excitation of carburized production gears,paired with carburized or hard-finished ones,would decrease if their total contact ratios have been set higher. The present study reveals that the exciting force is much dependent on the total contact ratio, and is little affected by chosing a integer contact ratio. And the exciting force of gear decreases at higher total contact ratio while no minimum force is observed.When a drive gear rotates at a fixed speed, the variation of driven gear teeth delays, from the theoretical tooth location determined from the gear ratio, due to the tooth deflection and profile errors. This delay is normally variable, and its variable amplitude, determined from the pitch circle of the gear, is denoted as the meshing transmission errors (hereinafter referred to MTEs), and the foundamental harmonic mesh component in the exciting force, can be expressed as the foundamental component of MTEs (2).Another finding from our study is that the MTEs of a gear pair decrease if the tooth face of the drive gear, where the gear tends to advance in the rotation, is smoothly ground off by a few microns. In other words, the MTEs component in the total tooth-face error has been successfully separated by analyzing delay or advance of the driven gear. Moreover,it has been clarified that such MTEs component of the error can be decreased.