It is very important for the design of composite columns to accurately evaluate the maximum depth-to-thickness ratio for which local buckling can be ignored and to determine the actual bearing capacities of slender columns which are influenced by local instabilities.This paper studied the behavior of bare steel tube and steel tube filled with concrete subjected to axial load.A total of 24 specimens were tested.The main parameter is breadth-to-thickness ratio,which varies from 50 to 125.Also,the finite element software Abaqus was applied to analyze the behavior of hollow steel tube filled with concrete,whose accuracy were validated by comparing with the corresponding experimental results.The results showed that the concrete in the steel tube can increase the buckling bearing capacity evidently with the increase of breadth-to-thickness ratio.