In this paper, the fluid- solid interaction in an electrostatic microbeam by using three- dimensional aerodynamic theory has been studied. Modified couple stress theory is used to model the elasticity depends on the size of the microbeam. The proposed model can be used as a mass micro- sensor. To analyze the dynamic behavior of the microbeam a DC voltage applied to the system and then by applying an AC voltage dynamic characteristics of the system around static deformed condition is analyzed. Because of non-linear nature of the governing equations to solve them reduced order model based on Galerkin is used. Results have shown that considering the couple stress and also increase the size of the length characteristic parameter reduces the size of the fluid pressure differential created between the two sides of the microbeam. However, according to the three- dimensional aerodynamic theory for fluid-solid interaction, change of the pressure difference created does not lead to creation difference in predicting the size of the added mass between the classical and modified couple stress theories. In another part of the results has been shown that the presence of added mass to what extent can makes changes in the frequency response curves drawn for the system. Also applied the couple stress theory and increase the size of the length characteristic parameter makes the system more rigid and consequently reduce the amplitude of the vibration and frequency response curves shift to the right.