This paper shows a detailed knowledge of how to design the tooth-surface-modification to realize a quiet helical gear pair where face width is finite in length. The performance of gears with bias-in and bias-out modification are discussed considering the effect of the shaft parallelism deviation. To clarify the effect of the tooth deviation types on the vibration behavior of helical gear pairs@ performance diagrams on vibration are introduced and created@ in which the acceleration level of gear pairs are shown by contour lines on the contact ratio domain.