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A single action rapid compression machine was developed to observe the soot formation and oxidation processes in a diesel spray flame. Two color method was applied to analyze the flame temperature and KL factor from the flame image taken by high speed camera. Variation in gas oxygen concentration of the surrounding gas was achieved by adding different quantities of pure oxygen, nitrogen, carbon dioxide and argon gases to charged air within a range from 17 to 25 vol.% oxygen to examine the effects of the surrounding gas composition and the temperature, and of the flame temperature on soot formation and extinction. The initial gas temperature has much effect not only on the ignition but on soot formation speed. The higher oxygen concentration gives the higher flame temperature and the faster soot oxidation rate in the flame. Carbon dioxide has a soot reduction effect in spite of its lower flame temperature.