Abstract: A method for determination of melamine in seawater and sediments was established by UPLC-MS/MS. Water samples were filtered， and extracted by Bond Elut Plexa ^TM PCX cartridges. Sediment samples were extracted with 1% trichloroacetic acid solution, and then cleaned with Bond Elut Plexa ^TM PCX cartridges. The analyte was separated with 5 mmol•L^-1 ammonium acetate (containing 0.1% formic acid)acetonitrile as mobile phase， and determined by UPLC-MS/MS. The limit of detection in sediments is 0.5 μg•kg^-1. When spiked 0.55.0 μg•kg^-1 in sediments, the mean recovery is over 70%, and the RSDs are less than 12%. The limit of detection in seawater is 1.0 ng•L^-1. When spiked 1.0-10.0 ng•L^-1 in seawater, the mean recovery is over 75%, and the RSDs are less than 10%.