Stepping tapping is a new tapping mode for Nickel-based superalloy GH4169. Longer life of tap and higher machining efficiency can be obtained. But chipping of tap is very prevalent. In this paper, the mechanism of chipping in step tapping is investigated. Based on the wavelet analysis of the signals of tapping torque, the load applied to the tap in every stage of the period of motion is studied. It is found that chipping is prone to occur at the moment when the rotating direction of tap shifts from forward to backward, which is caused by the too large and rapidly changed tensile stress on cutting edge. The excessive wear of cutting edge results in the too large tensile stress. To reduce the wear, the backward rotating angle of tap should be selected to be smaller than 100°.