The formation characteristics of NO_x during combustion of four typical Chinese coals have been studied in a packed bed combustor. The studies included the effects of chemical composition of coals and volatile matters, and combustion conditions, i.e. preheating temperatures of combustion gas, on conversion of NO_x during combustion. In term of the combustion experiments, there is a close relation between coal composition and conversions of NO_x. Coals with a higher nitrogen content give a lower F_NOx⁰, and coals with a higher volatile content give a lower F_NOx⁰. The further devolatilization experiments showed that volatile matters in the coal have an important effect on reducing conversion of coal-nitrogen to NO_x. It was interpreted as that the release of volatile matters during combustion caused a fuel-rich circumstance in the vicinity of coal particles which may restrain the formation of NO_x. Preheating temperatures of combustion gas has significant effects on conversions of NO_x and there exist a critical preheating temperature. At such temperature, lower F_NOx⁰ can be reached.