In the allowable stress design, steel members have to be secured a larger than normal strength and the compressed plate of steel than yield strength. Specially, structural members have to be secured strength and deformation capacity in plastic design or aseismic design. Therefore the estimation of structural members is basic and important. In order to find out the maximum strength and deformation capacity, the test and analysis are carried out. Specimens are stub-column and beam. A cross section used in test is cold formed square hollow section steel members and the parameter of specimen is Width-Thickness-Ratios. The purpose of this paper is to obtain limiting Width-Thickness-Ratios in accordance with a required deformation capacity in structural steel design.