Abstract: To investigate the relationship between the structure and catalytic activity, four types of MnO₂ nano-catalysts with various crystal structures (α-MnO₂, β-MnO₂, γ-MnO₂ and δ-MnO₂) were synthesized by hydrothermal method, and their low temperature NH₃-SCR activity were tested. The results indicated that catalysts with different structures showed various activities which followed the sequence of γ-MnO₂ > α-MnO₂ > β-MnO₂ > δ-MnO₂. It was found that γ-MnO₂ showed highest catalytic activity and its NO_x conversion rate surpassed 90% at the temperature range of 150-260℃. The catalysts were characterized by X-ray diffraction(XRD), scanning electron microscopy (SEM), N₂ adsorption-desorption, thermogravimetric(TG), infrared (FT-IR), temperature programmed reduction(H₂-TPR) and pyridine infrared spectroscopy (Py-FTIR). It was inferred that the morphology of the α-MnO₂ and β-MnO₂ were nanorods, while γ-MnO₂ and δ-MnO₂ with the structures of nanoneedles. The specific surface area of the catalyst was not the dominant factor affecting the NH₃-SCR activity at low temperature. The decent pore structure, strong redox property, abundant chemisorption oxygen and Lewis acid sites were responsible for high low temperature NH₃-SCR activity of γ-MnO₂ nano-catalyst.