Abstract: The stability constants of the complexes of Am(Ⅲ) and Cm(Ⅲ) with α-hydroxy-α-methyl valeric acid (α-HMVA) , determined by static cation exchange at 25℃ and ionic strength of 0.5(NaClO4), are found to be β1= (2.12±0.11)×102, β2=(1.71±0.19)×104, β3 = (4.99±0.70)×106 for Am(Ⅲ) complexes and β1 = (2.42±0.05)×102,β2=(2 .70±0.11 )×104, β3 = (9.27+++++++0.40) ×105 for Cm(Ⅲ) complexes. The separation factor of Am(Ⅲ) and Cm(Ⅲ) is 1.86(based on β3) . It is the highest value being reported with α-hydroxy carboxylic acid as a ligand.The constants of Am(Ⅲ) and Cm(Ⅲ) with seven a-hydroxy carboxylic acids are compiled. All data but those of glycolic acid have been determined in this laboratory. The effects of different alkyl-group structure on constants and separation factor are discussed.In the presence of α-hydroxy carboxylic acid, the separation factor of Pr(Ⅲ) and Nd(Ⅲ) which are similar to those of Am(Ⅲ) and Cm(Ⅲ) are investigated. From the figure of αprNd v·s αAmCm, the relation of αprNd=1.5αAmCm can be established.This relation can also be derived by the use of semi-experience and semi-theory formula. Therefore, it is possible to find the best ligand for separation of Am(Ⅲ) and Cm(Ⅲ) based on data of Pr(Ⅲ) and Nd(Ⅲ).