Bifurcation phenomenon of an elastic-plastic thin-walled tube which is subjected to internal pressure and axial tension is analysed as a variational problem formulated by Hill's sufficient condition for the uniqueness of solution. Proportional loading is assumed for the stress state in uniformly deformed pre-bifurcation state and a generalized Prandtl-Reuss equation is used as a constitutive equation for the elastic-plastic material. A characteristic equation which determines the bifurcation stress is derived and dependences of the bifurcation point for axisymmetric and non-symmetric modes of various orders on the stress ratio are discussed in some numerical examples.