The optimal adjacent amount range is an importance parameter in ultra precision microgroove machining of nickel phosphorous (Ni-P) plating mold for glass molding. This paper proposes a method by using small cross angle microgroove cutting to test the optimal adjacent amount range in the process of single point diamond cutting of microgrooves rapidly. In this method, the limits of the adjacent amount range is forecasted based on the similarity of the material removal form between the small cross angle microgroove cutting and the adjacent microgroove cutting. Firstly, the cross angle and microgroove depth are set as the microgroove cutting variables to perform experiments and to obtain several pairs of arrises . Then, the material removed state is analyzed based on the plastic deformation law and brittle-ductile transition theory through the observation of the gradient arrises. The section of removed material in gradient arris machining process is analyzed and compared with that of adjacent microgrooves, and their relationship is established. Finally, the differences between the exit arrises and the entrance arrises are analyzed and the adjacent amount range of the brittle-ductile transition area is determined. According to this method, the arris morphology is observed and calculated, it is found that the critical adjacent amount range of the brittle dissection phenomenal is 570-720 nm in ultra precision microgroove machining of nickel phosphorous (Ni-P) plating mold. The feasibility of the method is verified by ultraprecision machining of the brilliant microgroove mold. The mold is put into service in glass molding process and high precision optical glass microgrooves are manufactured.