Using a new extractant, N,N,N',N'-tetraoctyldiglycolamide (TODGA), the dependence of the distribution ratio on the ionic radii for M-II was investigated, and their relation with the structures of metal complexes were discussed. Ca-II (ionic radius: 100 pm) shows the highest D values, and the D values decrease with increase and decrease of ionic radius from 100 pm. The geometrical optimization by density functional theory (DFT) of M-II-TODGA complex suggests that the carbonyl oxygen has more affinity with metal than the ether oxygen, and the distance between the center of oxygen donor and the edge of cation corresponds to the van der Waals radius of oxygen atom. The most stable complex, as 1:1 complex, can be formed with Mg-II from DFT calculation, the complex becomes unstable with increase of ionic radius of M-II. This order is good agreement with the magnitude of D for Ca-II, Sr-II, and Ba-II.