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The static pull-in instability of the electrostatically actuated cantilever micro-beams incorporating Casimir force was investigated based on the modified couple stress theory. The nonlinear governing equation and boundary conditions were derived using the principle of minimum total potential energy. The pull-in voltages and non-dimensional pull-in displacements of the electrostatically actuated cantilever micro-beams were calculated by applying the generalized differential quadrature method（GDQM）. The results show that the pull-in voltages of the electrostatically actuated cantilever micro-beams based on the modified couple stress theory are greater than the classical results and the size effects are obvious. The presence of the Casimir force can reduce the pull-in voltages and non-dimensional pull-in displacements of the cantilever micro-beams and the influence of the Casimir force can be neglected when the initial gap of the cantilever micro-beams g is far greater than the thickness of the beam h.