Abstract: The effect of alkyl group structure of dialkyldithiophosphinic acid on separating Am(Ⅲ) from Eu(Ⅲ) is investigated. With four kinds of dialkyldithiophosphinic acids(HA) with different alkyl group of C 8(alkyl= n octyl, 1 methylheptyl,2 ethylhexyl,2,4,4 trimethylpentyl) as extractants and kerosene as diluent, the dependencies of extracting Am(Ⅲ) and Eu(Ⅲ) on pH and extractant concentration are determined. The slopes of lg D ～pH dependencies are close to 3 and the slopes of (lg D -3pH)～lg c ((HA) 2) are close to 2 except for di n octyldithiophosphinic acid extracting Eu(Ⅲ). While the organic phases are 0.2 mol/L extractants in kerosene and aqueous phase is 1 mol/L NaNO 3, the pH 1/2 values of aqueous phase for extracting tracer amount of 241 Am by HA with R= n octyl, 1 methylheptyl, 2 ethylhexyl, 2,4,4 trimethylpentyl are 2 31, 2 56, 2,58, 3 14, respectively, and the pH 1/2 values of aqueous phase for extracting tracer amount of 152,154 Eu(Ⅲ) by HA with R=1 methylheptyl, 2 ethylhexyl, 2,4,4 trimethylpentyl are 3 95,4 01, 4 42, respectively. The alkyl group has some effect on the extraction power to Am(Ⅲ) and Eu(Ⅲ), but has no substantial effect on SF Am/Eu.