Due to the toxic and un-dissolvable properties, the medical application of realgar (As 4 S 4 ) has always been limited for a long time. In this study, a novel method of dissolving realgar by immobilized Acidithiobacillus ferrooxidans ( A. ferrooxidans ) is introduced. The characterizations of dissolving realgar are carried out by means of SEM, ICP-AES, XRD and FTIR. The results suggest that the bio-transformation process is influenced by extracellular substances such as pH value and the oxidation of Fe 2 + . These extracellular substances affect the concentration of arsenic in bio-transformation solution rather than A. ferrooxidans . Present study reveals the novel application of A. ferrooxidans . [ABSTRACT FROM AUTHOR]