The paraffin wax was used as a diluent for 1-nitroso-2-naphthol(HA) and trioctylphosphine oxide(TOPO) in the extraction of lanthanides at 70℃. The composition of the extracted species was given as LnA3(TOPO)2 by means of the slope analysis. The variation of the synergistic extraction equilibrium constant(Ksex) was investigated at 60～80℃, and the thermodynamic data were calculated. The dependence of separation factors on temperature was also studied.