The fundamental characteristics and mechanism of extraction of cobalt (II) and nickel(II) by di-(1-methylheptyl) phosphinic acid (DMHPA) were studied. The extraction ability of cobalt(II), nickel (II) and various metals by DMHPA decreases in the order Fe(III) > Zn ≫ Pb(II) > Mn(II) > Cu(II) > Co(II) > Mg > Ca > Ni(II). The difference of the pH½values for nickel and cobalt reaches 2.06 pH units, which is apparently greater than those from extraction by di(2-ethylhexyl)phosphoric acid (DEHPA)and 2-ethyl-hexyl ester 2-ethylhexylphosphonic acid(DHEHPA).The slope analysis and IR spectroscopy reveal that the stoichiometrics of cobalt and nickel extracted species are Co(HA2)2and Ni(HA2)2(H2O)2respectively. As demonstrated by the study of the electronic spectroscopy the structure of the extracted complexes were shown as tetrahedral and octahedral configuration respectively. Furthermore, the coordination field parameters of the complexes were calculated.