Abstract: Mechanical behaviors and properties of carbon nanotubes are drawing increasing attention from the mechanics community. Both the “Bottom Up” approach based on quantum mechanics and the “Top Down” approach based on continuum mechanics are frequently used to model mechanical behaviors and properties of nanotubes. For some problems, a nanotube can be well described as a continuum solid cylindrical beam or shell. For some other problems, nanotubes show unique properties which cannot be described by continuum methods alone and require atomic or quantum descriptions of the phenomena involved. Hybrid Atomic/Continuum (HAC) approach has been recognized to be a useful tool to describe the effect of atomic details on the macro properties and behaviors of nano-structures and materials. In this paper, buckling of single-walled carbon nanotubes is modeled via a HAC approach. The model shows that zigzag nanotubes are stiffer than armchair tubes. The conclusion is validated by Tersoff-Brenner molecular dynamics simulation.