This paper aims to investigate the size effect on the free vibration responses of nanobeams with various boundary conditions, especially guide supported boundary conditions. It is seen that the boundary conditions examined in the previously published articles are mostly clamped-clamped, simply supported at both ends and clamped-simply supported. The difference of this article is that it examines the size effect based on the modified couple stress theory on vibrations of nanobeams with guide supported boundary conditions as well. In addition, the influences of the cross-section and the rotary inertia effect change on the vibrational responses of the nanobeams are pursued as a case study. A finite element method procedure is utilized to calculate the free vibrational frequencies of nanobeams.