A constitutive relation for a viscoelastic-plastic body is derived by employing a scalar and a tensorial internal variable that include viscous effects as well as plastic strain and the heardening parameter. Emphasis is placed on the characterization of the variables, which are closely related to creep strain, and the material parameters are determined from data from conventional tension and creep tests. The model is applied to the analysis of successive deformation of a thin-walled tube subjected to internal pressure under cyclic temperature variations. The correlation between the analytical and experimental results is discussed.