Methylmercury (CH3Hg) was known as an extreme toxicity and its ability to bio-accumulate in biota and to bio-magnify in aquatic food web. In this study, methylmercury in the marine sediment samples collected from the coastal area beside Dinh An estuarine (Hau river) in Soc Trang province was determined using the gas chromatography with MS detector. The study on selection of sediment samples preparation wasimplemented in order to get the better recovery efficiency. The established procedure of methylmercuryanalysis showed that the detection limit of method (MDL) and the quantification limit of method (MQL) were 21.8 µg/kg and 69.2 µg/kg, respectively. The recovery ranged from 80 to 102% with a relative standard deviation (RSD) lower than 15%, and the uncertainty was about 10.1% with the analysis of a certified reference materials (CRMs). The analytical method for CH3Hg is thus sufficiently accurate, reliable and applicable for the levels evaluation of CH3Hg in the marine sediment samples. The content of CH3Hg in the above mentioned samples ranged from 26.2 ± 2.6 to 48.1 ± 4.3 µg/kg.