Low NO_X combustion system was developed for gas firing. Most overseas gas turbines are required for firing oil as backup fuel. Low NO_X combustion systems for dual fuel firing are required. Lean fuel premix combustion is used for gas and lean fuel diffusion combustion for oil. Basic tests were conducted to optimize a premix fuel-air nozzle and performance was verified in high-pressure combustion tests. Commissioning tests on gas and oil burnig were completed at overseas power plants.