This paper studies the effect of surface oxidization on the normal spectral emissivity of straight carbon steel Q235 in air at 1.5 μm over the temperature range from 800 to 1100 K. For this reason, the normal spectral emissivity of straight carbon steel Q235 is measured at sixteen definite temperatures over a 6-hour heating period. The normal radiance emitted from the specimen is received by an InGaAs photodiode detector. The temperature of specimen surface is measured by the two platinum-rhodium thermocouples. The variation of normal spectral emissivity with the heating time is studied at a given temperature. The variation of normal spectral emissivity with the temperature is evaluated at a definite heating time. The strong oscillation of normal spectral emissivity is discussed, which is affirmed to be connected with the thickness of oxide layer on the specimen surface, and originate from the interference effect between the radiation stemming from the oxide layer on the specimen surface and the radiation coming from the underlying metal substrate. The uncertainty of normal spectral emissivity contributed only by the surface oxidization is about 1.5–7.1%, and the uncertainty of temperature generated only by the surface oxidization is about 1.7–5.8 K. The models between the normal spectral emissivity and the heating time or temperature are evaluated. A simple functional form including the exponential and logarithmic functions can be used to reproduce well the variation of normal spectral emissivity with the heating time at a given temperature, including the reproduction of strong oscillations.