On the performance of the hydrogen peroxide method for Th-determination we encountered some difficulties, namely the formation of the insoluble matter occured in the nearly neutral solution after evaporation. We carried out the Th-determination in the mixture of the Th and some kind of the rare earth, and found that the insoluble matter was not formed in case of the mixture of Th and some kind of rare earth, but formed in case of Monazite. The formation of the insoluble matter in the case of Monazite sand, however, could be avoid by using the beaker instead of the evaporating dish and by adding the water repeatedly before it was completely evaporated to dryness.