This paper re-defined the concept of conjugate axis and presented the application of this concept. Based on the research mentioned before, this paper studied the interference surface generated by the milling cutter, when a double circular gear is generated by a milling cutting tool. We fined a way to eliminate such the interference surface by the help of conjugate axis. The research of this paper gives the concept of conjugate axis the new function. The method presented in this paper is very useful for the grinding process of the hardened double circular gears.