The photodegradation of persistent and bioaccumulative perfluorooctanoic acid( PFOA)in water by185 nm vacuum ultraviolet (VUV) light was examined to develop an effective technology to deal with PFOA pollution. PFOA degraded very slowly under irradiation of 254 nm UV light. However, 61.7% of initial PFOA was degraded by185nm VUV light within 2h, and defluorination ratio reached17.1%. Pseudo first-order-kinetics well simulated its degradation and defluorination. Besides, fluoride ion formed in water, 4 shorter-chain perfluorinated carboxylic acids (PFCAs), that is, perfluoroheptanoic acid, perfluoorhexanoic acid, perfluoropentanoic acid, and perfluoorbutanoic acid. These were identified as intermediates by LC-MS measurement. These PFCAs consecutively formed and further degraded with irradiation time. According to the mass balance calculation, no other by products were formed. It was proposed that PFCAs initially are decarboxylated by185 nm light, and the radical thus formed reacts with water to form shorter-chain PFCA with one less CF2unit.