In order to clarify the factor related to the removal efficiency of nitrogenous compounds in supercritical water oxidation, the effects of molecular structure of nitrogenous compounds to be decomposed and organic substances as additives were investigated. The effect of molecular structure of nitrogenous compounds was examined by using ammonium carbonate, urea and formamide derivatives, and it was found that nitrogen removal efficiency tended to increase with the number of carbons bound to the nitrogen atom. A similar tendency was observed when methanol or other organic compound was added to the reaction system. The improvement of nitrogen removal efficiency was thought to be caused by the reaction of nitrogenous compounds with ammonia and radical species produced by the supercritical water oxidation of organic substances.