Microbridge tests were conducted on two sets of trilayer samples, consisting of SiO2/Si3N4/SiO2 and Si3N4/SiO2/Si3N4, which were fabricated on 4 in. p-type (100) silicon wafers by means of the microelectromechanical fabrication technique. The experimental results were analyzed using the small deformation formula including substrate deformation. By changing the sample length, we determined the bending stiffness and the resultant residual force per unit width in each of the trilayer thin films. A simplification is proposed to estimate residual stress and Young’s modulus in each layer. Young’s moduli and residual stresses are estimated to be 54.59 GPa and −429.49 MPa for the silicon oxide films and 270.54 GPa and 550.75 MPa for the silicon nitride films, respectively. The microbridge testing method enables us to characterize the mechanical properties of each layer of sandwich-structured thin films.