Degrading organic dyes via catalytic processes for waste water purification is an important research topic from the environmental conservation point of view. Herein, the catalytic performance of tungsten blue oxide (WO x ) nanoparticles was investigated systematically by varying the reduction temperature. The optimum reduction temperature to obtain the most stable WO x phase was obtained when plasma-synthesized WO3 nanoparticles were thermally reduced at 425 °C. The as-synthesized nanoparticles had an average diameter of 10 nm and a calculated band gap of 2.37 eV, which is lower than that of the WO3 nanoparticles (2.61 eV). The WO x nanoparticles exhibited an excellent performance in degrading rhodamine B under dark conditions and visible light irradiation, with a reaction rate constant 93 times higher than that of the WO3 nanoparticles.