The analytical method was developed for the speciation of 6 arsenic species [As(III), As(V), monomethylarsonic acid (MMA), dimethylarsinic acid (DMA), arsenobetaine (AsB), and arsenocholine (AsC)] in sea mustard (Undaria pinnatifida), dashima (Laminaria japonica), and laver (Porphyra tenera) of seaweed. After extraction under sonication with a solution of 50%(v/v) methanol in 1% $HNO_3$ and subsequent clean-up with anion-exchange cartridge, arsenic species were quantified by liquid chromatography with inductively coupled plasma-mass spectrometry. The ion-exchange cartridge is effective to remove a lot of interference. The gradient elution method was used for high resolution of arsenic species and the optimum pH of mobile phase is 6.0 and 6.2. Limit of detection and quantification were set at 0.06-0.12 and 0.20-0.40 ng/mL, respectively. Overall recoveries of arsenic species from fortified samples in seaweed were 81.1-118.8%, except for AsB and AsC in dashima and laver. This proposed method could be useful for quantification and speciation of arsenic species in seaweed as well as other foodstuffs.