Reactive ion etching of InP using a C2H6/H-2/O2 mixture was investigated. The effects of oxygen gas added to a C2H6/H-2 gas were systematically investigated. A very smooth etched surface was obtained by the addition of O2, even when the etching depth was 3.5 mum. Mass spectrum of the gases in the etching chamber was also investigated during etching. It was found that the O2 gas reduced the formation of hydrocarbon polymers in the etching process.