Abstract: A theory of hobbing involute conical gears on a hobber with a horizontal feed motion is described in the light of the gear engagement principle.The relations among some important parameters of the imaginary rack shaper cutter and the generated gear are analyzed,and the design method of a change speed train mainly composed of intersected involute conical gears is considered in detail.It has many special advantages such as many speed degrees only with a few gears,adjustable gear clearance by changing position of the gear along with their axes respectively.Because in designing this kind of train it is very important how to minimize the volumn and size and how to maximize the changable speed degrees and the difference between the highest degree and the lowest degree.A practical method is provided to control some key parameters to reach these goals.The present theory is proved correct and the method is shown practical by tests of a sample change speed train.