Abstract: In this paper, the author deduces the formulas of curvature of conjugate surfaces having two degrees of freedom by applying the method and mathematical tool used in his paper"Analy tical Theory of Space Meshing" The conditions of curvature interference and degeneration of these surfaces are also obtained A systematic discussion is made on the theory of hobbing and shaving of spur and helical gears. The author deduces an approximate formula to evaluate the profile errors between the works cut with the hob and corresponding rack-type cutter, etc. As the examples of practical application, the cases of hobbing of spur and helical gears with either involute hob or Archi median hob are discussed.