A study was carried out to determine arsenic species in Porphyra seaweed originating from the China Sea. Information about arsenic species in Porphyra was provided by HPLC-ICP-MS and ES-MS-MS. The total arsenic concentrations of Porphyra samples from five different producing areas ranged from 2.1 to 21.6 mg/kg. The analysis report also showed that arsenosugars were the only arsenic species that could be detected in all of the extracts of samples. Arsenosugar PO(4) was the major compound in most samples (0.3-13.9 mg/kg of dry weight), followed by arsenosugar OH (0.7-6.2 mg/kg of dry weight). A further experiment was done to investigate the stability of arsenosugars in the process of being heated. It was observed that the arsenosugars were stable during a short-term heating at 100 degrees C. Their stability in human ingestion was also studied. A substantial increase of dimethylarsinic acid (DMA) was detected in urine samples collected from six volunteers after the consumption of this seaweed. The results obtained indicated that arsenosugars had been metabolized to DMA, which is more toxic than arsenosugars. From this point of view, consumers should consider the possible adverse effects of edible Porphyra on human health and choose those Porphyra having lower arsenic concentrations.