Deformation and stress of a thick cylinder are analyzed by use of only a two-dimensional theory, not a three-dimensional one. Most thick cylinders actually used, for example, tubes in heat exchangers, are partially subjected to internal pressure in the longitudinal direction in the manufacturing stage, but no practical and applicable methods in which elastic-plastic deformation and yield strength of a thick cylinder are dealt with easily in three dimensions have been developed from the viewpoint of field application. In this work an applicable method based on thin cylinder theory and experimental results is proposed. This method yields results which are consistent with the behavior of thick cylinders, and enables one-step elastic-plastic analysis of double thick cylinders, for example, expanding tubes in heat exchangers.