: In order to industrialize gas sensors based on micro/nanomechanical beams, it is crucial to have a model capable of predicting the behavior of the system. The first part of this work deals with the size effect in micro/nanostructures. Experiments on micro/nano-mechanical systems (M/NEMS) have shown that their behavior under bending loads departs in many cases from the justify; unicode-bidi: embed; DIRECTION: ltr" Results showed that the size effect in micro/nanostructures should be considered so as to predict their behavior. It is revealed that the micro/nanofabrication processes may change the mechanical properties of the material. And also, for Aluminum, manufactured by the presented fabrication process, it is shown that the size effect is important when the size of the structures is scaled down to submicron. Moreover, in order to investigate the effects of gaseous molecules adsorption on the surface of micro/nanobeams in capacitive gas sensors, various methods of gas adsorption on a surface were investigated and then carried out with a model to predict the mechanism of the gas adsorption on a surface. The system was modeled using Matlab software and employing finite difference method to solve the adsorption equation. Finally, dynamic responses of the beam before and after gas adsorption were studied. The numerical analysis was performed using a Matlab code and Galerkin-based method to solve the non-linear equation of motion of the beam due to electrostatic force in capacitive sensors Keywords: size effect; Effective Young’s modulus; residual stre surface elasticity; length scale parameter, adsorption
