The photodegradation of organic dye rhodamine B (RhB) in an aqueous solution was studied using a micro-wave electrodeless UV lamp (MWUVL). The 185 nm vacuum UV (VUV) intensity of the MWUVL amounted to 12% of the total UV output, about 1.7 times as high as that of theconventional low-pressure mercury lamp. An acidic condi-tion was preferred for the degradation of RhB. Precipitate was generated during RhB degradation when the initial concentration of RhB solution was above 60 mg·L-1. For 100 mg·L-1 RhB at pH 4.0, the mass of precipitate produced was nearly half of the initial amount of RhB in the solution after irradiation for 75 min. The corresponding degradation ratio of RhB reached up to 99.4% and COD reduced 77.4%.The photodegradation of RhB is mainly achieved by direct photolysis of RhB by 185 nm and OH radical oxidation.