Due to necessary of existence of access opening, inspection paths, installation, entrance and exit doors, etc, creation of cutouts on the vessel structure is unavoidable. On the other hand, composite structures and structural analysis is complex and create cutout and imperfection structure increase these complexity. The aim of this research is to determine the mechanical response of three cutout positions on composite pressure vessels under 30 bar external pressure, so that no buckling and fracture failure occurs. Also, the optimum composite vessel thickness for this condition and cutout effect has been determined in this study. The studied vessels are made from E-Glass fiber and Epoxy matrix. Finite element simulation used to investigate the parameters effect. For this reason, commercial ABAQUS software and linear and non-linear analysis carried out to examine the parameters. To evaluate the simulation results, two composite vessels manufactured and fractured under external pressure. Moreover, the final vessel with three cutouts tested under 30 bar external pressure. The concluded results show that the optimum thickness was 16 mm for vessel with three cutouts and create the cutouts led to decrease buckling pressure. Also, with increasing cutout size the percent of buckling pressure increased.