To investigate in vitro dissolution of Arsenic from the ultra-micro and traditional powder of Realgar.Methods:AFS (atomic fluorescence spectrophotometry) was used to detect the dissolution amount and rate of Arsenic from the ultra-micro and traditional powder of Realgar.Results:The dissolution amount of Arsenic from the ultra-micro powder is 2.79 times larger than that from traditional powder, and the values of Td and T0.5 of Ultra-Micro powder were significantly lower than those of the traditional powder.Conclusion:ultra-micro powder of Realgar is superior in dissolution amount and rate over traditional powder.