Consideration of dynamic and static behavior of structures in nano and micro scale for analysis and predicting of their performance and accuracy have more importance. In this study, the effect of size and intermolecular van der Waals force on dynamic behavior of torsional nanomirror considering bending-torsion two degree of freedom model using the higher order modified couple stress theory has been investigated. First considering the higher order modified couple stress theory and intermolecular van der Waals force, equation of motion of system is developed, afterwards using Rung-Kuta method, this equations is solved and dynamic performance of nanomirror and its phase portraits have been obtained. Also translational and torsional natural frequencies of system considering applied voltage are investigated. So pull-in instability parameters of system are considered and their dependency upon van der Waals force and size effects are determined. Results demonstrate that equilibrium points of system include center points and focus points that phase portraits related to these points exhibit periodic orbits and heteroclinic orbits. Also size effect and modified couple stress model on amplitude and frequency of vibration of system have been investigated. Proposed model in this study is able to predict experimental results with higher precision than previous classic models and reduce the difference between past theories and empirical results.