Abstract: In studying the substoichiometric extraction of thorium with l-phenyl-3-methyl-4-benzoyl-pyrazolone-5 (PMBP), it was found that thorium can be extracted substoichiometri- cally by 5.0×10-4M PMBP-xylene solution in the range of pH 4.5-5.5. Thorium in synthetic mixture solution and ore samples was determined by substoichiometric isotope dilution method and the preliminary results were presented.