Ion-pair reverse-phase HPLC–inductively coupled plasma (ICP) MS was employed to determine arsenite [As (III)], dimethyl arsenic acid (DMA), monomethyl arsenic (MMA) and arsenate [As (V)] in Chinese brake fern (Pteris vittata L.). The separation was performed on a reverse-phase C18 column (Haisil 100) by using a mobile phase containing 10mM hexadecyltrimethyl ammonium bromide (CTAB) as ion-pairing reagent, 20mM ammonium phosphate buffer and 2% methanol at pH 6.0. The detection limits of arsenic species with HPLC-ICP-MS were 0.5, 0.4, 0.3 and 1.8 ppb of arsenic for As (III), DMA, MMA, and As (V), respectively. MMA has been shown for the first time to experimentally convert to DMA in the Chinese brake fern, indicating that Chinese brake fern can convert MMA to DMA by methylation.