A rapid analysis method is proposed to distinguish between gas oil and fuel oil discharged into sea area. The proposed analysis was performed on a FT-IR spectrometer. Because those oils could be distinguished from three peaks at 811, 742 and 723 cm⁻¹, they were used as discrimination indices. Those peaks are assigned to CH bending modes for 2 and 4 hydrogens and methylene framework, respectively. As a result, the absorption intensities of gas oil was 742<811<723 cm⁻¹, but those of the fuel oil was 723<742<811 cm⁻¹. The absorption of 1603 cm⁻¹ was larger in fuel oil than gas oil. And the absorption of 475 cm⁻¹ was not observed in gas oil, while it was in fuel oil. Because the frequency intensities were peculiar to the oil types, the spectra offered the rapid type analysis of the discharged fuel oil and gas oil. However, the frequencies of 2955, 2925, 2854, 1462 and 1377 cm⁻¹ of the gas oil were hardly distinguished from those of the fuel oil. This method was effective for the analysis of the discharged gas oil and fuel oil into sea areas.