Abstract: Allinger's force field (MM2) is used to study the structural effect of alkyldialkylphosphonates on uranium and thorium extraction. The interactions betweenmetal ion and ligands are treated as charge interactions. The calculated structuresof the complexes of UO_2(NO_3)_2 and Th(NO_3)_4 with water molecules areconsistent with the experimental data. According to the differences of stericenergis (ΔE_s) between alkyl dialkylphosphonates and their metal complexes, theextraction ability of alkyl dialkylphosphonates is chiefly governed by the stericeffect of the alkyl group. But there exist some other factors such as entropyeffect which also influenced the extraction. Moleculat mechanics calculationsreveal that in the uranium complexes the s-butyl estet groups make the torsionalenergy much lower than the others, so that the s-butyl ester group is veryfavourable to the uranium extraction. The steric effect of alkyl groups is reflectedon the congestion between the ligands and therefore decreases the extraction abilityof the reagents.