Strength and stiffness of locally deformed wall of CHS (circular hollow section) column connected to beam flange in the absence of diaphragm were experimentally investigated. Compressive and tensile loads applied to the cylindrical wall via the flange are associated by almost the same yield strength as well as elastic stiffness. Maximum strength, however, is much higher in tension than in compression, which is accounted for by the generation of tension field in large plastic deformation. On the basis of the experiment, a simple mechanical model is introduced, from which formulae for calculating yield strength, plastic strength, ultimate strength, and elastic stiffness are developed. Their predictabilities are examined by comparing the predictions with experimental data of this study and past ones of other researchers.