A micro scale nonlinear beam model based on strain gradient elasticity is developed.Governing equations of motion and boundary conditions are obtained in a variational framework.As an example,the nonlinear vibration of microbeams is analyzed.In a beam having a thickness to length parameter ratio close to unity,the strain gradient effect on increasing the natural frequency is predominant.By increasing the beam thickness,this effect decreases and geometric nonlinearity plays the main role on increasing the natural frequency.For some specific ratios,both geometric nonlinearity and size effects have a significant role on increasing the natural frequency. A micro scale nonlinear beam model based on strain curvature elasticity is developed. Governing equations of motion and boundary conditions are obtained in a variational framework. As a example, the nonlinear vibration of microbeams is analyzed. A beam having a thickness to length parameter ratio close to unity, the strain gradient on on natural frequency is predominant.By increasing the beam frequency, this effect decreases and geometric nonlinearity plays the main role on increasing the natural frequency. For some specific ratios, both geometric nonlinearity and size effects have a significant role on increasing the natural frequency.