When the a graphite crystal is crushed, it is breaks into micro-scale laminar flakes. There are many dangling bonds on the edge of an isolated graphite micro-sheet which elevates its energy and makes it unstable. When a graphite micro-sheet curles into a nano-tube, the number of dangling bonds decrease, and the system energy is reduced accordingly. Meanwhile, the curled sheet obtains deformation energy, which can more than counteract the energy reduction due to the diminished dangling bonds and make the CNT even less favorable energetically than the sheet, and becomes unstable. On the basis of advancing the mechanical model of the formation of CNTs, a thorough theoretical analysis was made, which lead to the conclusion that the CNTs can exist stably at a minimum diameter of approximate 0.32nm.