Abstract: Microscale beam exhibits size effect and this can be described by strain gradient theory. The dynamic model of bilayered micro-beams is developed using equivalent relationship of monolayered and bilayered beams based on the modified couple stress theory. To illustrate the new model, the free vibration problems of a simple supported beam are solved, and the influences of characteristic size and bimaterial parameters on inherent property of bilayered beam are analyzed. Numerical results reveal that the natural frequency increases significantly compared with the classical one when the thickness of beam is comparable to the material length scale parameter. The natural frequency, however, is close to that of the classical theory as the thickness of beam is far greater than the material length scale parameter. The dimensionless natural frequency of bilayered beams exhibits size effect and there is a little difference among them when the bimaterial parameters change. Bilayered micro-beams can be simplified to monolayered micro-beams when the thickness of one layer is much thicker than that of the other layer.