We have proposed a cold plastic working method, in which a partial diameter expansion part necessary to boss or gear stop can be formed on the thick walled pipe under multiple stress conditions combined cyclic torsional stress and axial-compressive stress. The diameter expansion deformation can progress easily with mechanical ratchet phenomenon generated by the alternate stress, using the method. In the present study, diameter expansion processing tests for the pipe were carried out under various processing conditions by using the new processing machine developed originally. Diameter expansion deformation behaviors on the processed part were measured continuously, and diameter expansion rates were calculated based on the deformation behavior. Furthermore, simulation analyses of the processing were done by finite element method (FEM). It is clarified experimentally and analytically that the diameter expansion deformation behaviors and the diameter expansion rate change according to processing conditions.