In the present study, critical bending buckling moment of perfect and defected single-walled carbon nanotubes (SWCNTs) have been investigated using structural mechanics approach. Nonlinear finite element analysis is designed for determination of critical bending buckling moment and postbuckling behaviour of SWCTs. For applying pure bending moment, a new displacement-control procedure based on large deformation theory is employed to the ends of the SWCNT incrementally. By doing so, moment-curvature diagram could be derived during pure bending loading. In case applying moment reaches a critical value, bending buckling onset of SWCNT is appeared and critical bending moment and critical bending curvature is determined. In the following, the postbuckling behaviour of defected and perfect SWCTs is revealed. The effects of SWCNT aspect ratio (length/diameter) and defect types (vacancies and Stone-Wales) on the critical bending moment and critical bending curvature are studied for zigzag nanotubes. For perfect SWCNTs, results have good agreement with those in the literature. Also, it is found that vacancies defect decreases the critical bending moment more than Stone-Walls defect.