This paper deals with the effects of carefully controlled rotary truing operation on fracture of grain cutting edges of CBN wheel. Truing forces(cutting forces in truing operation) are measured experimentally. An assessment of the resulting wheel performance during the grinding of a brass workpiece and an observation of SEM micrograph of grain cutting edges on wheel surface is reported. The generation mechanism of grain cutting edges is discussed. The main conclusions obtained in this study are summarized as follows: (1) There exists two main forms of fracture of CBN grains that can be divided crushing and attrited wear. Then the two forms can also be controlled finally by selective conditions such as the truing speed-ratio, sliding speed etc.. This can be used as a guide for selection of truing conditions. (2) There is fine relationship between normal truing force and normal grinding force, and this relation is independent of the truing speed-ratio and the truing sliding speed.