This paper deals with buckling strength of axially loaded cylinders considering interaction between initial imperfection and plasticity. We conducted 14 axial buckling tests of large-scale fabricated steel cylinders in the 1980s to investigate the effects of yield stress and length of cylinder on buckling strength. Then, we found the discrepancies between the test results and the predictions by the existing buckling design codes like the buckling design guideline of DnV for marine structures and the code case of ASME for nuclear containment vessels and recognized that it was important to consider the interactive effect of initial imperfection and plasticity. We apply the reduced stiffness theory by Dr. J. G. A. Croll and show a simple prediction method of buckling strength considering interaction between initial imperfection and plasticity. The predictions by the reduced stiffness theory give a good agreement with results of bucking tests and buckling analysis using nonlinear finite element method. Then, we verify the validity of the reduced stiffness theory in the wide range of dimensions of cylinders.