A study of chemical oxygen-iodine laser (COIL) for the use of decommissioning and dismantlement of nuclear facilities is conducted. A scaled-down model was developed as a prototype. Laser duct and optical cavity were designed so that it can be operated in both supersonic mode and high-pressure subsonic mode for the comparative study. A 1.34kW output with chemical efficiency of 24.6% was obtained in the supersonic mode. In the high-pressure subsonic mode, output power was 1.12kW with chemical efficiency of 20.6%. A subsonic operation at 12Torr was demonstrated for the first time. A preliminary experiment of thick steel cutting was demonstrated by the developed system. The obtained data was in good agreement with published data.