α-MnO2 nanoparticles with the average size about 22-29 nm were synthesized by hydro- thermal method. The as-synthesized samples were characterized by X-ray diffraction (XRD), Fourier transform-Infrared Rediation (FT-IR) and thermogravimetric-differential thermal analysis (TG- DTA). We find that the average particle size of α-MnO2 is increased with the reaction temperature increasing. The diluent reactant is beneficial for the growth of α-MnO2. The smaller the molar ratio of K/Mn is, the more stable α-MnO2 phase is. The Neel temperature (TN) of nano-α-MnO2 is 5.0K obtained by the magnetic susceptibility determination, lower then the value of 24.5K for MnO2 single crystal phase. The magnetic hysteresis loop can be observed obviously at T = 5.0 K. It indicated that when the particles are at the level of nano-size level, the anti-ferromagnetic MnO2 showed the ferromagnetic nature at the lower temperature. When the temperature is high up to 60.0K, the magnetic hysteresis loop can't be observed. While when the temperature is at 120.0K, a straight line of magnetic hysteresis loop is obtained, indicating the paramagnetic magnetic nature of our sample.