Although many researchers investigated the effect of geometrical imperfection on the buckling load of unstiffened shells, the stiffened shells were not studied yet. In this paper, the effects of geometrical imperfection the buckling load of unstiffened and stiffened composite shell with and without cutout are investigated. For this goal, several specimens are manufactured and tested. The mechanical properties of fibers and resin matrix and volume fraction of fibers in the shell and the stiffeners are determined based on the standard tests. Finally, the mechanical properties of each component are calculated by micromechanical relations. These properties are used for finite element modeling by ABAQUS package. Linear eigen value analysis and nonlinear RIKS method -which can consider the geometrical imperfection- are used. FE results are validated in comparison with experimental tests. Using FE model, the effects of imperfection amplitude on the buckling behavior of unstiffened and stiffened shell with and without cutout are studied. The results show that geometrical imperfections have more effect on the buckling load of unstiffened shells in comparison with stiffened ones. Nevertheless, ignoring these imperfections and using eigen value analysis overestimate the buckling load. This fact is more evidence for shells without an opening. In perforated shells, the cutout itself represents an imperfection that is much more significant than geometric imperfections.