Perfluorooctanoic acid (PFOA) is a new-found hazardous persistent organic pollutant,and it is resistant to decomposition by hydroxyl radical (HO-) due to its stable chemical structure and the high electronegativity of fluorine.Photocatalytic reduction of PFOA with β-Ga2O3 in anoxic aqueous solution was investigated for the first time,and the results showed that the photoinduced electron (e-cb)coming from the β-Ga2O3 conduction band was the major degradation substance for PFOA,and shorter-chain perfluorinated carboxylic acids (PFCAs,CnF2n+1COOH,1 ≤ n ≤ 6) were the dominant products.Furthermore,the concentration of F- was measured bythe IC technique and defluorination efficiency was calculated.After 3 hr,the photocatalytic degradation efficiency was 98.8％ and defluorination efficiency was 31.6％ in the presence of thiosulfate and bubbling N2.The degradation reaction followed first-order kinetics (k =0.0239 min-1,t1/2 =0.48 hr).PFCAs (CnF2n+1COOH,1 ≤ n ≤ 7) were detected and measured by LC-MS and LC-MS/MS methods.It was deduced that the probable photocatalytic degradation mechanism involves e-cb attacking the carboxyl of CnF2n+1COOH,resulting in decarboxylation and the generation of CnF2n+（1）.The produced CnF2n+（1） reacted with H2O,forming CnF2n+1OH,then CnF2n+1OH underwent HF loss and hydrolysis to form CnF2n+1COOH.