Concrete filled tubular steel columns, called CFT columns, where concrete is used to fill round or square steel tubes, are increasingly adopted in civil engineering works of Japanese Railways. Especially, after Hyougoken-Nanbu Earthquake which attacked the western part of Japan in 1995 with serious damage done to a large number of reinforced concrete railway viaducts, the high seismic performance of CFT columns attracts attention of civil engineers. On the other hand, when steel tube and concrete are treated as a composite structural element, the evaluation of its bearing capacity and ductility is not yet established. Then, a series of alternate load tests with CFT column models simulating railway structures were carried out. As a result, an excellent ductility of CFT columns under constant axial compressive load and alternating bending was demonstrated. In this paper, based on the above-mentioned experimental results, a method of quantitatively evaluating the relation between ductility and section parameters of CFT columns was developed.