A new method, in which a piston is formed from thick plate of aluminum-silicon alloy under hot working, has been developed by authors. Namely, in the first process a blank is formed into a shape of piston having a bearing part of a piston-pin by drawing, ironing and bottoming. In the second process a bearing part in the piston is made by means of burring. Finally, the plastically formed piston is finished by machining for grooves of piston rings and an oil ring. In this paper, the forming method of the piston of internal combustion engine is mainly described. Furthermore, each stress distribution of plastically formed piston and die-casting one is examined by strain gage method and simulation of Finite Element Method. As a result of comparing both pistons, there are scarcely problems in practical use for this plastically formed piston.