This paper describes the bearing strength of concrete filling in a steel column. Single or double tubular column specimens filled with concrete that have mechanical shear keys, were tested under compressive and tensile load. The test results verified that the width to thickness ratio of column influenced on the bearing strength of concrete filling in a steel column, and the influence of number of the mechanical shear keys, embedded depth of the inside column and eccentricity of the inside column were small. Local stress states near the mechanical shear keys were modeled and the yield condition by Mohr-Coulomb applied to the stress state. Based on the model, the equations of evaluating the bearing strength were derived and its validity was confirmed.