Mechanisms and characteristics of NOx formation/destruction in laminar diffusion combustion of coal-gasified fuel in air are investigated by numerical computations taking into account detailed chemical kinetics and multicomponent diffusion. Conclusions were as follows : (l) Fuel N (NH3) is decomposed to NHi by OH, H and O in the fuel-rich region which results in NO formation. (2) CH4 contained in the fuel induces HCN formation in the fuel-rich region. HCN is decomposed in a region closer to the stoichiometeric condition as compared with NH3. HCN is converted to NO which increases the conversion rate of NH3 to NOx compared with the case when HCN is not generated during combustion. (3) Effects of NH3 and CH4 contents in the fuel, the preheating temperature and the excess air ratio were investigated to reveal the NOx formation characteristics of the fuel.