Bending performances of aluminum square tube beams reinforced by aluminum plates under three point bending loads have been evaluated using experimental tests combined with theoretical and finite element analyses. Basic properties of aluminum materials used for initial input data of the finite element simulation were obtained from the true stress-true strain curves of specimens which had bean extracted from the Al tube beams. True stresses were determined from applied loads and cross-sectional area records of a tensile specimen with a rectangular cross-section by real-time photographing. True strains were obtained from in-situ local elongation measurements of the specimen gage portion by the multi-point scanning laser extensometer. Four kinds of aluminum tube beam specimens adhered by aluminum plates were employed. The bending deformation behaviors up to the maximum load described by the numerical simulation were in good agreement with experimental ones. An aluminum tube beam strengthened by aluminum plate on the upper web showed an excellent bending capability.