In the plastic bending of sheet metals,spring back is well-known phenomenon,but contains conplicated and difficult problems. Spring back of thin sheet metal has a serious effect upon the finish of bending materials, but students major in technology education hardly understand this phenomenon. Throughout the experimental bending tests of thin metals with the V-die and the punch, the author dealt the process of deformation and obtained the amount of spring back in the plastic bending of three types of sheet metals -galvanized steel sheets, aluminum sheets and brass sheets having several thicknesses. The results showed as follows: 1) Spring back occurs at the bending of thin metal sheets. 2) Aluminum sheets is almost not occurred the spring back phenomenon. 3) The thickness of metal sheets mainly influenced to spring back. As the thickness of the sheet decreases, the amount of spring back increases. 4) Spring back is influenced by the radius of the punch.