With water as an extractant,a method was established for the determination of furoic acid in transformer oil by reversed phase liquid-liquid microextraction(RLLME) coupled with high performance liquid chromatography(HPLC).The effects of some important parameters,such as pH value and amount of extractant,amount of the diluent(n-hexane),extraction time and centrifugal rotational speed were investigated.The optimum experimental conditions were as follows:extraction solvent:neutral ultrapure water;amount of extractant:150 μL;ratio of dilution:Voil∶Vn-hexane=3∶1;extraction time:5 min;centrifugal rotational speed:5 500 r/min.The quantitative analysis of furoic acid in the transformer oil sample was performed by HPLC.The calibration curve was linear in the range of 0.05-5.00 mg?L-1 with a correlation coefficient(r)of 0.999 9 and a detection limit of 9.6 μg?L-1.The relative standard deviation(RSD,n=6) was 1.4%and the enrichment factor of furoic acid was 13.6.The recoveries of furoic acid in aging transformer oil sample ranged from 99.7%to 105.1%.The qualitative analysis of furoic acid in aging transformer oil was further carried out by using HPLC-MS.This study has established a simple and effective method for the detection of furoic acid in transformer oil and provided useful data for the research on the aging characteristic parameters of transformer oil-paper insulation.