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Three kinds of dialkylphosphinic acids (DAPAs), i.e. dihexylphophinic acid (DHPA), di-(2,4,4-trimethylpentyl) phosphinic acid (DTMPPA) and didecylphophinic acid (DDPA), were synthesized through free radical addition reaction. The influence of the types of initiator, reation time and reaction temperature on the yield of DAPAs were investigated. The products were characterized by NMR and MS. By using DHPA, DTMPPA and DDPA (10% in kerosene) as extractants, the extraction of Co2+ and Ni2+ in sulphate medium at different equilibrium pH values were measured. The results show that the maximum yield of DHPA, DTMPPA and DDPA can all be achieved at about 130 ℃ under the initiation of di-tert-butyl peroxide (DTBP). All the extraction of cobalt with respect to DHPA, DDPA and DTMPPA precedes that of nickel. The difference in pH1/2 value (defined as the pH at which 50% metal extraction occurs) between cobalt and nickel increases in the following sequence from large to small: DHPA, DDPA and DTMPPA, which indicates that the separation ability for cobalt and nickel ascends from DHPA, DDPA to DTMPPA.