The lead-cooled fast reactor is considered one of the promising Generation IV nuclear energy systems. Structural materials used in the construction of pressure vessels and internals for this reactor include 300 series austenitic stainless steels. Nb-containing austenitic stainless steels are developed to improve corrosion properties, mechanical properties, and irradiation resistance. However, coarse primary NbC carbides are formed during solidification in these steels and cannot be eliminated through subsequent hot working and heat treatment. Recently, researchers have found different oxidation behaviors between secondary phase particles and the matrix, which affect the material's corrosion properties. However, the oxidation behaviors of primary NbC are rarely reported. This study analyzes the corrosion behaviors of a solution-treated Nb-containing austenitic stainless steel plate after exposure to oxygen-saturated liquid lead-bismuth eutectic (LBE) at 550 and 600 ^oC using SEM, EPMA, XRD, and TEM. The results show that the oxidation probability of NbC is correlated with its location in the samples at 550 ^oC. NbC at the initial surface is easily oxidized, while NbC within the interior is difficult to oxidize due to the low equilibrium oxygen partial pressure in the inner oxide layer, which suppresses the oxidation of NbC. However, NbC at the initial surface and within the interior are prone to be oxidized as the temperature increases to 600 ^oC. Compared to the matrix, NbC oxidizes into Nb₂O₅, resulting in a higher Pilling-Bedworth ratio (PBR). This leads to high compressive stress and resultant microcrack formation in the surrounding oxide layer. Additionally, the presence of CO₂ generated during the oxidation of NbC within the interior reduces the compactness of the oxide layer, leading to a higher growth rate.