Abstract: An ultra-low level liquid scintillation spectrometer (Quantulus GCT 6220) was applied for the simultaneous determination of ³³P and ³²P in environmental samples. The energy ranges for ³³P and ³²P and the experimental conditions were optimized. The results showed that counting efficiency was high when volume ratio of cocktail to sample solution was greater than 2, volume of mixed solution was greater than 12 mL, dark time was greater than 6 h, and pH was 6~7. The counting range of ³²P was selected to be 150~1710 keV and that of ³³P (replaced by ¹⁴C) to be 0~150 keV, which could eliminate the interference between the simultaneous measurement of ³³P and ³²P, with efficiencies of 54.8%±8.7% for ³³P and 64.7%±3.7% for ³²P. Based on this method, the lower limits detections were obtained, and environmental samples were measured.