The purpose of this thesis is buckling and post-buckling analysis of the cold-formed sections. The cold-formed sections have small width-to-thickness and because of their two-dimensional geometry do not fail in the buckling stress. So the study of post-buckling behaviour of these sections is an important subject in mechanic science. The exact solution of equations governing post-buckling behaviour is complicated. So Finite Strip method has used. This method is rapid and has high precision in analysis of folded plates. In this thesis local, distortional and global buckling of thin-walled sections are studied, while post-local buckling of these sections that have initial imperfection is investigated. Considering of the initial imperfection, we can follow the load-displacement path, because the shape of initial imperfection is based on initial buckling. Buckling and post-buckling analysis is studied under different loading. The buckling behaviour of different sections under uniformly distributed load is investigated. The effect of varying thickness in cross-section has also investigated in buckling stress and post-buckling stiffness. Investigating of buckling behaviour of different sections results local buckling stress of the sections under moment is greater than the stress of sections under pure compression. The sections under uniformly distributed loads buckle in a greater stress rather than the sections under pure bending. In the process of determining of post-buckling stiffness it has found out that each section has greatest stiffness in a special dimension. The stress redistribution of sections changes with changing of dimensions. Keywords : post-buckling, cold-formed sections, finite strip, initial imperfection.