Abstract: The synergistic extraction of rare earth ions (Ⅲ) (Ln3 + =La3+ ,Pr3+ ,Eu3+ , Gd3+, Tb3+, Er3+, Yb3+, and Y3+) from hydrochloric acid solution by mixtures of l-phenyl-3-methyl-4-benzoyl-pyrazolone-5 (HPMBP)and primary amine N1928,(R1R2 CHNH2, RNH2)have been investigated in xylene at 25±1℃. The resultsshow that Ln(Ⅲ) in the form of Ln(PMBP)3 in HPMBP solution alone arc extracted into organic phase under the experimental conditions.It is found that the synergistic coefficients vary with atomic number of rare earth elements and the "double-peak effect" is demonstrated. The mechanism of the synergistic extraction with HPMBP-N1923 are determined by the methods of slope and constant mole. The synergistic species RNH3Ln(PMBP)4 is found to be extracted into the organic phase. The synergistic extraction reactions and addition reactions of the synergistic extracting complexes in organic phase are proposed as follows.Ln3+ + 4HPMBP(0)+RNH3Cl(0)RNH3Ln(PMBP)4(0)+H++Cl-Ln(PMBP)3(0)+HPMBP(0)+RNH3Cl(0) RNH3Ln(PMBP),2(0)+H++Cl-In addition, the equilibrium constants of the synergistic extraction reactions and addition reactions of praseodymium (Ⅲ) are calculated to be logK12= - 1.95, logβ12 = 3.94. The IR and NMR spectra of the synergistic extracting complexes of praseodymium(Ⅲ)are studied as well.