This work proposes a new approach to characterize the mechanical properties of nanowires based on a combination of nanomechanical measurements and models. Silicon nanowires with a critical dimension of 90 nm and a length of 8 mu m obtained through a monolithic process are characterized through in-situ three-point bending tests. A nonlinear nanomechanical model is developed to evaluate the mechanical behavior of nanowires. In this model, the intrinsic stress and surface parameters are examined based on Raman spectroscopy measurements and molecular dynamics simulations, respectively. This work demonstrates a new approach to measure the mechanical properties of Si nanowires by considering the surface effect and intrinsic stresses. The presented technique can be used to address the existing discrepancies between numerical estimations and experimental measurements on the modulus of elasticity of silicon nanowires.