The nitrided MoO3 catalysts formed using two kinds of treatment with either NH3 or He after nitriding were studied by temperature-programmed desorption and X-ray diffraction analyses. The catalysts were cooled to room temperature in either flowing NH3 or He (NH or HE catalyst) after nitriding at 773, 973 and 1173 K with NH3. The activities of the catalysts were determined during the hydrodenitrogenation of carbazole at 573 K and 10.1 MPa total pressure. MoO2, gamma-Mo2N, and Mo metal were mainly formed in the NH catalysts nitrided at 773, 973 and 1173 K, respectively. Mo oxides and metals in the NH catalysts were nitrided to gamma-Mo2N and beta-Mo2N0.78 With low crystallinity during TPD. The surface area of the NH and HE catalysts nitrided at 773 K increased to 66 and 59 m(2) g(-1) maximum from 1.1 m(2) g(-1) of fresh MoO3, respectively, but decreased as the nitriding temperature increased to 973 K and 1173 K. The HE catalysts per surface area were more active than the NH catalysts for both the overall HDN reaction and hydrogenation, and the 1173 K-nitrided catalysts were highest. On the other hand, the NH catalysts were more active than the HE catalysts for C-N hydrogenolysis and the 973 K-nitrided catalyst showed a maximum activity for C-N hydrogenolysis.