Abstract: A gas chromatographic method using capillary chromatographic column (OV-17) and electron capture detection was proposed for the determination of methylmercury in biological samples.The sample was pre-treated with 5 g·L-1 aqueous solution of L-cysteine assisted by microwave heating,to extract methylmercury and make it dissolved in aqueous solution,which was back-extracted with benzene after acidifying the solution with HCl,to separate methylmercury from fat and protein co-existing in the sample.The range of linearity for the method was from 0.1-1.0 ng,with a detection limit of 0.064 ng.Precision test at the concentration level of 0.80 ng gave a value of RSD (n=7) of 4.58%.In its application to the analysis of several aquatic products recoveries in the range of 95.5%-102.3% were obtained.