According to the analysis of engagement between indexing roller and cam contour as gear indexing cam mechanism operating and milling process as contour being machined, three kinds of surfaces, cam’s contour, the surface swept by roller’s axis during engagement and the surface swept by cutter’s axis in milling process, are equidistant surfaces. Based on isometric theory, the approaches to determine tool path and to calculate NC program coordinates for inequi-diameter milling have been developed. The architecture of computer aided programming software for inequi-diameter milling was illustrated. Practical example of inequi-diameter milling NC codes was presented. In inequi-milling process, NC program can be updated to adapt the change of cutter diameter in time. So, cutter’s life can be prolonged, the cost of cutter can be cut down.