Relaxation of internal stress for micro-electromechanical systems (MEMS) using SiO2 / Al2O3 / SiO2 membrane has been studied. The aluminum oxide thin films were formed by electron beam evaporation at room temperature. No peaks were observed in the X-ray diffraction pattern of the films. The ratio of Al and O in aluminum oxide was stoichiometric compared with Al2O3 target material. The internal stress was tensile at about 300-400 MPa. Bottom SiO2 thin film was formed by thermal oxidation and the top one by RF magnetron sputtering method. The internal stress of thermally oxidized SiO2 film was compressive at about 440 MPa, while that of the films deposited by sputtering, was compressive at about 100 MPa. The ratio of Si and O in each SiO2 thin films remained stoichiometric. The total stress of the membrane was controlled by optimizing the thickness of each film for relaxing the total stress of the membrane. The total stress of the membrane became almost zero under optimum conditions of SiO2 and Al2O3 films.