Abstract: A simple and sensitive method for separation of MMA, DMA, As(Ⅲ) and As(Ⅴ) is presented. Inorganic arsenic MMA, DMA, As(Ⅲ) and As(Ⅴ) are separated by the use of coprecipitation with Fe(OH) 3, cation exchange chromatography and extraction combined with neutron activation analysis. The recovery for MMA, DMA, As(Ⅲ) and As(Ⅴ) is 96.4%, 103%,96.3% and 104%,respectively. The applicability of the method to the arsenic species in a fresh water is demo strated. The detection limit of the method is 0.02 μg/L of arsenic. The results show that the arsenic concentrations in drinking water of Akebameng range from 204 μg/L to 1125 μg/L, which are much higher than the permitted limit(<50 μg/L). The dominant species of As in drinking water are inorganic arsenic compounds of which occupy 94%～99% of the total arsenic. The concentrations of MMA and DMA in drinking water vary from 4.61 μg/L to 20.7 μg/L and 5.69～ 18.2 μg/L , of which the ratio of MMA and DMA to total arsenic ranges from 1% to 6%.