In material science and engineering the fatigue of metal is quite difficult to deal with and that of welded joint is an even more complex phenomenon. This complexity arises from different main factors: thermal cycles during welding, which will strongly affect the base material and induce residual stresses and distortion, the fusion process with the filler metal leads to heterogeneity to microstructure which will affect or change the original mechanical properties and chemical composition through out of the welded joint. In this study isothermal mechanical fatigue of welded reactor steel has been contemplated. The samples of the welded joint have used in this study is 15H2MFA steel type of VVER-440 reactor pressure vessel operating at Paks Nuclear Power Plant, Hungary. The reactor vessel is made of low-alloy heat resistant steel and austenitic steel with a corrosion-resistant inner (clad) surface. Reactor pressure vessel (RPV) made of 2 ¼ Cr -1Mo-V bainitic steel having a good mechanical strength and creep resistance.