Abstract: This paper used FT-IR to analysisthe before and after oxidation of oil and the infrared absorption band of the gear oil was located in 1 724—1 684 cm-1. By analyzing the sulfur and phosphorus type additives，the wave number ranges of C—S groups were located in 1 130—1 100 cm-1 and 920—870 cm-1，P—O—C groups in 1 130—1 090 cm-1 and P=S groups in 920—870 cm-1. By using the IR spectrometer，the decay law of gear oil（GL-5 75w-90）which used in locomotive wasstudied。The results show that the changes of oxidation were rapidly increased within 100 000 km，and changed smoothly after 100 000 km. The degradation of extreme pressure-antiwear additives was mainly the degradation of phosphorus type additives，and the P=S groupsplayed a major role.

Key words: FT-IR, oxidation, degradation of additives