Abstract: An analysis method was established for the determination of As(Ⅲ) and total arsenic in stream sediments by Hydride Generation-Atomic Fluorescence Spectrometry (HG-AFS) by using 4.8 mol/L HCl for ultrasonic extraction at 50℃. The amount of As(Ⅴ) could be obtained by substracting As(Ⅲ) from total arsenic. The masking effects of citric acid, sodium citrate, oxalic acid, sodium oxalate, tartaric acid and sodium tartrate on As(Ⅴ) were investigated, then As(Ⅲ) was determined by controlling suitable acidity of medium. The results showed that the best masking effect on As(Ⅴ) is sodium citrate. Moreover, As(Ⅲ) is of higher sensitivity in the medium of sodium citrate. The optimum determination results could be achieved under the condition of ultrasonic extraction for 30 min, sodium citrate for 6 g/L and determination acidity for 0.24 mol/L. Meanwhile, the signal interference of As(Ⅲ) on As(Ⅴ) can be greatly reduced. The detection limit is 0.41 μg/L and the relative standard deviation level is 1.33% (n=11). The linear range of the calibration curve is 1.5-500 μg/L. The recoveries of As(Ⅲ) and As(Ⅴ) are 82.7%-95.3% and 96.1%-107.6%, respectively. The method is simple, rapid, highly accurate and of low cost. It has been applied to determine the As(Ⅲ) and As(Ⅴ) in stream sediment samples with satisfactory results which meet the requirements of multiple geological sample analysis.