Abstract: In this paper, the possibility of complexometric titration of microgram amounts of actinide using highly sensitive Tb3+-Tiron fluorescence complex to judge the equivalence point is put forward.With thorium as an example, the optimum conditions for the determination of a few micrograms of thorium are investigated, and an unusual complexometric procedure is established. The relative standard deviation for the determination of 1-2 micrograms of thorium is less than ±1%, while that for 4-5 micrograms of thorium is less than ?0.5%.The unusual complexometric procedure is checked by the results of determination of thorium on two monazite standard samples. Both accuracy and precision are within 0.5%.