Quantitative evaluation of oxidation behavior of high-temperature materials is imperative for applications in a Silicon carbide (SiC)-based thermal protection system (TPS) of reentry space vehicles. However, the reported oxidation rates obtained using thermogravimetric analysis (TGA) have been widely varied among researchers. In this study, this variation is assumed to be attributable to differences in the oxygen partial pressure at the sample surface due to different configuration of the experimental devices. Existing data are usually given as a function of the surface temperature of a specimen TS with the partial pressure of oxygen in the input gas (PO2)I as a main parameter. However, even if (PO2)I and TS are set to the same, the oxygen partial pressure on specimen surface (PO2)S and gas temperature TG are expected to vary widely, respectively, from (PO2)I, TS, and according to the difference in the oxidative environments. To evaluate these effects in various apparatuses, numerical calculations were conducted for the case of SiC. From these results, (PO2)S was found to be main source of the variation in oxidation rates.