In the article the theory of teeth profile formation and the equations of micro-segment rack & gear are introduced in brief, and the reasons that there exists zero relative curvature at many contact points of micro-segment involute profiles in engagement of concave/convex pattern with very small contact stress are analysed. In fact, the contact stress of the micro-segment involute gear is less than that of common involute gear. The inflexion of micro-segment gear compared with that of common involute gear via finite element method is introduced briefly. It is also pointed out why the micro-segment involute gear is more excellent than common involute one in the aspect of inflexion strength.