The elastic-plastic finite element analyses of the internal pressure vessels with the different ratios of the diameter of nozzle to the diameter of vessel are carried out to investigate the stress concentration in the nozzle-vessel junction. The results show that the heaviest stress concentration is found at the interval nozzle-vessel junction of the longitudinal symmetry plane, while the stress concentration around the external junction is decrease because of the welded joint. In the process of loading, the stress around the junction linearly increases at first and then the range of the high stress area enlarges. The ratio of the diameter of nozzle to the diameter of vessel has greater effect on the distribution area of high stress in plastic deformation stage. Compared with large opening without nozzles, the stress concentrations around the junction are sharply reduced because of the nozzles.