The chemical speciation of elements has been important because their bioactivity and toxicity rely on the particular chemical forms of elements. In all the arsenic species inorganic arsenic (As3+ and As5+) has been reported to be more toxic than organic species. This study was focused on the analytical methods for arsenic species in seafood and marine sediment samples. Five main species (As3+, As5+, MMA, DMA and AsB) were investigated (inorganic species for sediment sample only). The extraction method for seafood was chosen with sonication in which a water : methanol mixture (1:1, v/v) was used as the extract solvent. And for marine sediment samples, the extraction of inorganic species was carried out by shaking using 0.5 M phosphoric acid as extract solvent. The species were separated by a Hamilton X100 analytical column, and detected by a mass spectrometry. Various certified reference materials (CRM) were used for the validation of the analytical methods. For seafood CRM sample about 90 % of AsB the main species in seafood was extracted by the method mentioned. And 85.7 % and 82.2 % recovery efficiencies for As3+ and As5+ spiked in marine sediment CRM sample were obtained using the extraction mentod.