This study tries to investigate the reduction of nitric oxide by thermally cracked hydrocarbons under rich condition during diesel combustion. Experiments using flow reactor system, which follows the chemical process of fuel at high temperature and atmospheric pressure, show that thermal cracking of fuel starts at about 1000K, and lower hydrocarbons mainly composed of C₂H₄ and CH₄ are formed. NO can be reduced when fuel is thermally cracked and oxidized. A larger amount of NO is reduced when thermal cracking hydrocarbons are increased in quantity under rich and high temperature condition. Among decomposed hydrocarbons, C₂H₄ is easily decomposed and affects deNO mechanism. Chemical kinetic calculation using CHEMKIN III reveals the mechanism. NO is reduced through the reaction of HCCO or CH₂ with NO. In these reaction paths, C₂H₂ is an essential species. The computation also shows that this deNO mechanism can be actualized in the practical diesel combustion.