As an important raw material for synthesis of phosphor and laser gain medium, the sesquioxide of praseodymium (Pr₂O₃) receives few attentions due to its susceptibility to change of valence and hygroscopicity in air. Here, the reduction process from Pr₆O₁₁ to Pr₂O₃ and corresponding mechanism in air and Ar/H₂ atmosphere as well as the crystal phases, morphologies, particle sizes and valence states of the two kinds of oxides were investigated. Furthermore, the photoluminescent properties of praseodymium oxide were analyzed in relation to the valence state of praseodymium. The results show that phase transition of Pr₆O₁₁ significantly varied in different atmospheres. Reduction of Pr₆O₁₁ is accelerated in Ar/H₂ atmosphere and Pr₂O₃ can be obtained at 800 ℃. In addition to adsorption in ultraviolet region resulted from transition from valence to conductive band, the Pr₂O₃ containing Pr³⁺ also shows the absorption in visible light band caused by the f→f transition. Pr₆O₁₁ shows strong absorption to UV-Vis wavelengths over 320 nm, which is related to the charge transfer between Pr⁴⁺ and oxygen ion. Wide band in the fluorescence emission spectrum of Pr₂O₃ indicates that the lowest energy level of 4f5d orbital of Pr³⁺ is below ¹S₀. Fluorescence intensity of Pr₆O₁₁ containing Pr⁴⁺ decreases 63% compared to that of Pr₂O₃ at 404 nm, which can be attributed to the energy dissipation between Pr³⁺and Pr⁴⁺. This difference in fluorescence property can be used to analyze Pr valence in ceramics or crystals with high oxygen ion mobility. This research demonstrated that the transformation process and the related property from Pr₆O₁₁ to Pr₂O₃ in different atmosphere may promote the application of Pr₂O₃.