In this experiment, a gravity sedimentation method like Andreasen pipette methodl for the determination of the particle size distribution of thorium oxide is used. The weighing method of Andreasen Pipette Method for the accumulative percentage of the particles is replaced with a γ-ray counting method. The sample material was activated by irradiation in JRR-1 or labelled with radioactive thorium-234 (UX₁). In both cases, the particle size distribution of thorium oxide in the range of 0.3μ, to 70μ was determined. The principle and discussion of this method and the application are also described.