The principle of NC incremental sheet metal forming process as well as the experiment of cone forming are presented. Because the deformation of sheet metal only occurs around the tool head and the deformed region is subjected to stretch deformation, the deformed region of sheet metal thins, and surface area increases. Sheet metal forming stepwise is to lead to the whole sheet metal deformation. The experiment results show that in the case of the parallel line type tool path, a uniform thickness of the deformed region is maintained and in good accordance with that obtained by the sine law. It is found that success in the forming depends on the forming half-apex angleθof the truncated cone. It can be obtained that NC incremental sheet metal forming process is a plane deform process and conforms to the sine law, i.e.t=t0 sinQ.