In this research, the design of a cubic vacuum vessel is investigated. The design of these vacuum vessels is important, because a weak design may lead to a disastrous fracture or a very bulky and expensive vessel.The displacement equations of a 6×5×4 m cubic vessel under vacuum pressure is derived theoretically, assuming that the vessel is made of thin surfaces and the displacements are not large, while no rib is included in the design. The resultant maximum displacement from this method is calculated as 0.6 m, which is not acceptable and shows that the vessel needs to be strengthened using longitudinal and transversal ribs. By using ribs on different sides of the vacuum vessel, the bending strength is excelled. However since the geometry of the structure would become very complicated and non-linear, analyzing of the vessel using theoretical equations is very hard. Therefore the FE method is used to investigate the best combination of the rib parameters and vessel body thickness. It has to be noted that in FE simulations, since there are different solvers, mesh types, mesh sizes etc., the results vary from case to case. Therefore when solving a problem with FE method, the results have to be verified. The problem that arises however is that our current vacuum vessel is in the design phase and therefore the FE results can not be verified with experimental results. To solve this problem, a cubic vacuum vessel which is located in Isfahan Optic Complex is investigated and the experimental results are derived. In order to do this, the displacements of 15 points are measured under the vacuum condition. Although the dimensions of this vessel are different from our vessel, it can be used to verify the FE solution of our problem. This vessel is simulated in ABAQUS 6.9 and the results are in a good agreement with those achieved from the experimental tests. The 6×5×4 m vacuum vessel is simulated in ABAQUS with different combinations of rib height, rib lay out and vessel ody thickness. The FE parameters are chosen to be the same as those used in the previous phase. The design criteria is based on three facts: the minimum weight of the structure, the stresses and displacements that have to be in the allowable range. The safety factor is chosen to be 3 and the maximum displacement has to be 1 mm (as indicated in the literature). Five different vessels are investigated using FE method. The results showed that although the reduction of the vessel body thickness increases the stress and displacement, the amount of the weight increase is more crucial. The increase in the rib's height also affects the displacement amount significantly. Model 4 is selected as the optimum vessel in the sense of stress, strain and weight. Key words: Cubic vacuum vessel, ribs, FE simulation