Abstract: There are several methods in use for simplification of solving problems of crossed-axis helical gearing, but they all seem to 1.e simplified not basially. With auxiliary rack as a medium, meshing the helical gear with the rack, this paper studies first the conjugate action on the transverse plane and calculate (Xp, Yp) of the point on transverse path of contact out of (Xp, Yp, Zp) of the point on space path of contact. Then studies the normal contact condition of rack and helical gear in the plane normal to the trace of rack and passing through pitch point in order to get Zp Consequently, we get the three-dimensional coordinates of contact point in fixed space as follows: Zp = Xptan β1=h1λ (1) 1-Xp=Yptanγ(2) Where, γ=gradient angle in transverse plane of auxiliary rack profile meshed with given helical gear 1,β1, hl,λ1=helical angle, reduced pitch, helical rotation' angle of gear1. If gear 1is a spur gear the problem is reduced to a plane gearing. If gear I is a helical gear, we must calculate Zp andλ1 with the above formulae. Formulae 1and 2 are proven in detail on this paper. After determine (Xp, Yp,Zp) of the contact point, with the aid of screw motion and coordinate transformation, we can calculate the profile of mating helical gear in a very simple way. A hob design problem is used as an example and have been proven exactly by actual cutting. The main aim of this paper is to calculate the hob profile for helical gear work, such as hob for cycloidal helical pump gears. This kind of problem is rather complex. For example, Dr. Yu. V. Tsvis puts Zp=O for simplification in solving conjugate equation of hob for straight-profiled helical spline, so makes the spatial crossed helical gearing problem reduced to a plane one, hence a wrong result is obtained. This paper not only determines the point on path of contact in fixed space but also the point on line of contact on the rack-tooth surface with the same method.