A simple, easy-to-use and reliable method called nano-bending method is proposed to measure the residual stresses of surface-micromachined structures for MEMS (Micro-Electro-Mechanical Systems). The basic concept is based on the fact that the linear bending stiffness of the surface-micromachined bridge is affected by the residual stress of the consisting materials. It is preferentially necessary to know anchor stiffness and bending stiffness of the surface-micromachined cantilevers in the stress free state which are composed of the same materials as the surface-micromachined bridges. The usefulness of the method was demonstrated by poly-silicon surface-micromachined structures under various residual stresses. The results indicated that the measured residual stress of the as-deposited poly-silicon film is 142.70 MPa with standard deviation of 3.5 percent and the compressive residual stress decreases as growing the annealing effect.