The nanomechanical resonator is a useful structure to achieve various nano-electromechanical systems (NEMS) sensing devices. In this study, systematic evaluations of the dynamic properties and mechanical material properties of an ultra-thin nanomechanical resonator made of diamond-like carbon (DLC) were carried out. As a result, the fabrication of an ultra-thin nanomechanial resonator with a thickness of 4.8–73 nm was achieved by focused-ion-beam (FIB)-based nanofabrication. The energy dispersion of vibration depended on the surface area/volume (S/V) ratio. A thinner nanomechanical resonator had higher sensitivity to pressure. Furthermore, we evaluated Young's modulus and density as mechanical material properties by measuring the resonant properties of the DLC/SiO2 bilayer mechanical resonator. Young's modulus and density increased with decreasing thickness. This implied that the mechanical properties of nanoscale-thick DLC have a thickness dependence.