In respect to the composite beam-column which is made of H-shaped steel and concrete placed between upper and lower flange of the steel, the followings have been comprehended from experimental results on its plastic mechanical behavior and its hystresis characteristics against cyclic increasing bending moment under applying of constant axial force. 1.Concrete-crash does not reduce stiffening effect by concrete which is recognized in plastic mechanical behavior and hysteresis characteristics ; however, concrete does not almost bear the applying load after it is crushed. 2.In case of that the parameter a(=√<(t_f/b)(t_w/d)> is larger, state of flange in the tensile side becomes to be more remarkably inelastic. 3.Plastic deformation capacity is evaluated by the parameter α, which can be also quantitatively estimated in the safety side by the proposed equation.