A solid phase extraction coupled with high performance liquid chromatographic（HPLC） method was developed for the determination of trace amounts of mesotrione in water.The pH values of the samples were initially adjusted to 3.0-4.0 with phosphoric acid，and then enriched and purified with a C18 solid-phase extraction column.The mesotrione fraction was eluted with acetonitrile-water（1∶1） and then concentrated to 10 mL.Mesotrione was determined by HPLC equipped with a Thermo Hypersil-C18 column（250 mm×4.6 mm，5 μm），by using a mixture of acetonitrile-phosphoric acid solution（45∶55，pH 3.0） as mobile phase at a constant flow of 0.5 mL/min.The column temperature was set at 30 ℃ and the wavelength of ultraviolet（UV） detection was set at 233 nm.The experimental results showed that a good linear relationship（r＞0.999 7） between peak area and mesotrione concentration was obtained in the range of 0.4-20 μg?L-1.The method detection limit for mesotrione was 0.039 μg?L-1.The average recoveries were in the range of 90.3%-94.7% at spiked levels of 1，5,10 μg?L-1 with relative standard deviations of 3.5%-4.6%.The method was applied in the determination of trace amounts of mesotrione in surface water and wastewater with recoveries of 89.9%-93.2% at spiked amounts of 5 μg?L-1 and 10 μg?L-1.The method showed the advantages of simplicity， rapidness and sensitivity，and was suitable for the determination of trace amounts of mesotrione in water.