Abstarct: Equal Channel Angular Pressing (ECAP) is one of the severe plastic deformation methods which can be utilized to mechanical strengthening of mextals through the ultrafined grain structure. In this present research ECAP of a mextallic tube subjected to an inner pressure was investigated using upper bound and finite element method. In order to keep the circularity of the section during the ECAP of mextallic tubes, it is necessary to apply a pressure on the inner surface of the tube. For this purpose, different types of mandrel such as mextallic, polyurethane, grease and sand have been employed in the last researches. In the analytical section, firstly the mathematical relations for ECAP of a solid rod was presented by upper bound approach. Then, the similar formula for ECAP of a hollow tube by using a mextallic mandrel was achieved. Finally, the needed force for ECAP of a tube subjected to the inner pressure was investigated. The Bezier curves were used to obtain the deformation field, strain and strain rate tensors. A rigid-perfect plastic material as well as isotropic property was considered. The sticking friction model was used between the rigid die and the tube. Simulation of the ECAP of a tubular sample was performed by Abaqus/Explicit solver and the numerical results were compared with the analytical outcomes obtained from different inner pressures and tube thicknesses. The results showed that by increasing the inner pressure, the unwanted distortion of the tube section decreases while the ECAP load rises. This effect will increase by decreasing the tube thickness. Moreover, by decreasing the tube thickness in a constant inner pressure, the section distortion will rise and the ECAP load decreases. The results showed that by increasing the inner pressure, the magnitude of equivalent plastic strain will increase about 12.5%. Also, when the tube thickness decreases 34%, the required forming force decreases 33.3%