The objective of this research is to develop a new lapping process which can efficiently make tooth flanks of hardened steel gears smooth as a mirror. This lapping process is carried out on the trial-made gear lapping machine using a nylon helical gear as a lap. In this paper the lapping process accompanied by vibration of the nylon gear was attempted to improve the tooth roughness and the accuracy of hobbed gears. Micro hardness, micro structure and residual stress on the tooth flank of lapped gears were also examined. As the results, it was found that the vibration of the nylon gear increases the removal rate and reduces more efficiently tooth flank roughness of work gears. And the running noise and the load carrying capacity of lapped gears were investigated. The running noise and the load carrying capacity of lapped gears were improved.