This paper investigates the pull-in instability of a nano-switch under electrostatic and intermolecular Casimir forces. The analysis is based on the geometrically non-linear Euler-Bernoulli beam theory with consideration of the surface energy. Through differential quadrature method (DQM), the pull-in voltages of the nano-switch are obtained. Results show that the effect of surface energy and geometrically non-linear deformation on the pull-in voltage depends on the length, height and initial gap of the nano-switch. In addition, the effect of intermolecular Casimir force on the pull-in voltage weakens as the initial gap increases.