An experimental study of thermal DeNO(x) process with different additives was performed in an electricity-heated tubular flow reactor, showing that CO is less effective to lower the optimum temperature than H-2 and CH4 The maximum NO reduction is lowered with H-2 added, while it is hardly affected by CO or CH4. The temperature window is widened appreciably with CH4 added, while it is narrowed slightly by H-2 or CO The disadvantage of CH4 is that it causes CO emission due to its incomplete oxidation, and the maximum conversion of CH4 to CO is more than 50% In general, the calculation using a detailed chemical kinetic model predicts most of the process features reasonably well The analysis on reaction mechanism shows that the effects of these additives on NO reduction are achieved principally by promoting the production of OH radical