α-, β-, γ- and δ-MnO₂ catalysts were prepared by a hydrothermal method and their catalytic properties for NO oxidation were evaluated. There were dramatic differences in the activities among the MnO₂ catalysts with different crystal structures. The γ-MnO₂ catalyst exhibited the best activity among the four catalysts and gave rise to 91% NO conversion at 250 °C. Moreover, 50% of NO conversion was achieved by γ-MnO₂ even at 160 °C. To investigate the factors influencing the catalytic activity, the catalysts were then characterized by X-ray diffraction, N₂ adsorption, scanning electron microscopy, Fourier transform infrared spectrometry, Raman spectrometry, temperature-programmed reduction by H₂ and X-ray photoelectron spectroscopy. On the basis of the characterization results, the tunnel structure and surface chemisorbed oxygen of γ-MnO₂ were proposed to be the main factors that contributed to the excellent performance in NO oxidation. In addition, the four catalysts showed stable catalytic activity. Over γ-MnO₂, H₂O had a reversible inhibiting effect on catalytic activity, while the conversion of NO could not recover after removing SO₂. In the presence of H₂O and SO₂, H₂O was able to slow the progression of catalyst deactivation resulting from SO₂.