A method for the detection of trace As (III), As (V), monomethylarsonic acid (MMA), dimethylarsinic acid (DMA), arsenocholine (AsC) and arsenobetaine (AsB) in seaweeds by capillary electrophoresis-inductively coupled plasma mass spectrometry (CE-ICP-MS) was developed. The optimized conditions of CE including chemical component, pH and concentration of the buffer, separation voltage and injection time were studied in detail. The results showed that the above six species of arsenic were baseline separated within 25 min under the optimized conditions. The relative standard deviations (n = 6) of the peak areas were smaller than 5%, and the detection limits (calculated as As) (S/N = 3) were 0.08 - 0.12 microg/L for all the above six species of arsenic. The method has been successfully applied to the determination of the species of arsenic in kelp samples. The recoveries were 90% - 103%. The method has good reproducibility, high sensitivity, short analytical time and strong resistance to complex matrix, and can be used for the speciation analysis of arsenic in real seafood samples.