Ion implantation without any thermal treatment is applied for the residual stress relaxation of LPCVD (low-pressure chemical vapor deposition) polysilicon films in MEMS. He+ and Ar+ ion implantations reduce the residual stress of polysilicon films. The amount of residual stress relaxation increases as ion dose increases. TEM (transmission electron microscopy) observations show that ion implantation of polysilicon films changes the crystal state of polysilicon into an amorphous state. The residual stress relaxation of LPCVD polysilicon film in MEMS (micro-electro-mechanical systems) is attributed to the compressive stress created by the cubical expansion of polysilicon because amorphous silicon has a lower density than crystal silicon. This compressive stress counterbalances the tensile stress in the upper part of films with a positive stress gradient. The property modifications of polysilicon films by ion implantation are also investigated. The elastic modulus and hardness of polysilicon films with ion implantation is evaluated by the nanoindentation method. Ion implantation at an ion dose of 1016 ions/cm2 decreases the elastic modulus and hardness of polysilicon films. However, as ion dose increases, the elastic modulus and the hardness of polysilicon films increase.