Abstract: An approach for calculating tooth working-surface contact and tooth edge contact of modified helical gears is proposed.The generation of tooth surface and Tooth Contact Analysis(TCA)of the modified helical gears under various errors can be simulated on computer by the aid of this approach.Then,the tooth modification by changing tooth profile of rack-cutter and by CNC machine is considered.As an example,TCA on pair of modified helical gears is completed by using above metioned approach,and the improvement for contact under errors is shown.This method can be used for modification of helical gears.