Due to the widespread use of pressure vessels in various industries such as refineries, petrochemical industries, oil and gas, power generation, it is important to analyze these vessels in terms of temperature endurance and load. In many applications the vessel experiences different types of uniform and cyclic pressure or temperature loads. Therefore, the effect of temperature and pressure individually and/or in combination, on the vessel, is important. In many applications, the effects of pressure and temperature are usually considered by applying compressive pre-stress on the vessel. The materials used in the manufacture of these vessels usually are alloy steels, such as nickel-chrome with high strength. The material considered in this study with such strength was DIN 1.6959 steel. Stress analysis in the cylinder in the area of elastic and plastic was carried out under operating temperatures and pressures. The stress distribution through the cylinder thickness obtained in three load types including temperature, pressure and the combination of them, then the results are compared. Although the average value of Mises stress in the case of combination load is more than temperature load case, stress gradient in the thickness of cylinder is much stronger and the value of Mises stress on the inner surface is higher in the latter case. As a result, it can be concluded that sequence of temperature and pressure load is important in the system. For the validation of simulation of transient heat transfer with finite element software a semi-analytical solution was used to achieve transient stress and temperature distribution. A good agreement was observed between the semi-analytical and FE simulation results. To model the real behavior of material and considering Bauschinger effect, different linear and nonlinear hardening models were investigated in this study. Then, simulation of ratcheting in the cylindrical pressure vessel with nonlinear kinematic and isotropic hardening was performed. To use these non-linear mixed models, UMAT subroutines were used in simulations, and the simulation results of these subroutines used in the material model, were compared with experimental results of the same material in published articles for validation. Heat treatment was performed on the test specimens DIN 1.6959 and grips (grips material was considered hot work steel DIN 1.2344). For doing heat treatment on the test specimens, salt bath was used. To obtain material properties, tensile test samples of investigated material were prepared and then tension tests were performed in four different temperatures. To perform cyclic tension-compression tests at elevated temperatures, a back-lash free high-temperature tension-compression test equipment was designed and built. Then, tension-compression tests were conducted on the material DIN 1.6959. After doing heat treatment and cyclic tests on the samples, it was observed that the material softening occurs on the specimens. Isotropic and kinematic hardening parameters obtained according to abaqus help procedure, and simulation of vessels under cyclic temperature and pressure load performed with these material constant. Due to non-proportional loading on the vessel, as a result of temperature, pressure, and temperature-pressure operation on the vessel geometry, according to the stress and strain charts obtained, it can be concluded that the ratcheting occurs in the pressure vessels. Keywords: Thick Pressure Vessel, DIN 1.6959, Inside Pressure, Finite Element Simulation, Ratcheting Phenomena, Non-linear Combined Hardening, Subroutine UMAT, Thermo-Mechanical Test