KEYWORDSAbstract. In this paper, we study static bending and free vibration behaviors of Bernoulli-Euler microbeams with a single delamination using the modi ed couple stress theory. The delaminated beam is modeled by four interconnected sub-beams using the delamination zone as their boundaries. The free and constrained mode theories are utilized to model the interaction of delamination surfaces in the damaged area. The continuity as well as compatibility conditions are satis ed between the neighboring sub-beams. After veri cation of the results for some case studies with available solutions, the e ects of various parameters, such as spanwise and thicknesswise locations of the delamination, material length-scale parameter, and boundary conditions, on the static bending and free vibration characteristics of the size-dependent microbeam, have been investigated in detail.