This letter presents the application of electrostatic pull-in instability to study the size-dependent effective Young's Modulus Ẽ ͑ϳ170-70 GPa͒ of ͓110͔ silicon nanocantilevers ͑thickness ϳ1019-40 nm͒. The presented approach shows substantial advantages over the previous methods used for characterization of nanoelectromechanical systems behaviors. The Ẽ is retrieved from the pull-in voltage of the structure via the electromechanical coupled equation, with a typical error of Յ12%, much less than previous work in the field. Measurement results show a strong size-dependence of Ẽ . The approach is simple and reproducible for various dimensions and can be extended to the characterization of nanobeams and nanowires.