The oxidation kinetics and microstructure of S30815 heat resistant stainless steel， both in its base material and welded joint， are analyzed at different service temperatures by the constant temperature oxidation method. High temperature oxidation kinetic curves are obtained by the mass gain method. The morphology， composition， and microstructure of the oxide films are studied by using scanning electron microscopy （SEM）， energy dispersive spectroscopy （EDS）， and X-ray diffraction （XRD）， respectively. The results show that under constant temperature oxidation conditions at 780 ℃， there is less mass gain and a relatively lower oxidation rate. The oxidation products are mainly Cr₂O₃， Fe₂O₃， and Fe₃Mn₃O₈， exhibit sheet， strip， and polyhedron shapes. At 880 ℃， the mass gain and the oxidation rate significantly increase. The oxidation product at this temperature comprises a mixed oxide of Cr₂O₃， Fe₃O₄， MnFe₂O₄， and Ni （Cr₂O₄）， which is mainly in thin strips and sheets. The oxidation kinetics curves of S30815 follow a parabolic rule. With the increase of oxidation time， the oxidation rate gradually decreases and eventually tends to be stable， showing a good oxidation resistance at high temperature. A larger amount of dense Cr₂O₃ oxide film is generated on the surface of the welded joint， exhibiting a lower average oxidation rate and better oxidation resistance compared to the base material.