Autofrettage is the process of residual stress formation on the walls of the thick walled cylinders before their usage. These residual stresses help to increase the pressure bearing capacity of the thick walled cylinders by eliminating some stresses when service pressure is applied in the high pressure applications. In practice, there are two different autofrettage methods as hydraulic and swage. Swage autofrettage is a more economical method to form beneficial residual stresses in the thick walled cylinders when it is compared with the hydraulic autofrettage method. In the current study, swage autofrettage process, applied on to two thick walled cylinders which have gained different material properties by heat treatment process is investigated. The main objective of the study is to determine the optimum mandrel-inner diameter interference ratios that results in maximum benefit. In accordance with this purpose, it is determined equivalent stresses on elastic-plastic junction, which is the most critical region of autofrettaged cylinder, being assumed to be used in max. 400 MPa service pressure. At first, maximum Tresca and Von Mises equivalent stresses on the elastic-plastic junction are obtained analytically for the autofrettaged thick walled cylinders exposing to the internal pressure of 400 MPa. Then, the verification of the analytical model is performed by making use of a ABAQUS software which uses the finite element method. Swage autofrettage is applied in the factory considering optimum values and permanent expansion of inner and outer diameter is measured. It is also calculated making use of ABAQUS software and results are compared.