Monazite is rare earths phosphate mineral and includes radioactive mineral such as uranium and thorium. In order to use the sodium phosphate effectively produced from the caustic leaching process of monazite as by-product, the radioactive materials must be removed perfectly. In this paper, it was solved that radioactive colloids contained in sodium phosphate produced as a by-product in caustic digestion process of monazite are uranium pseudo-colloid and thorium hydroxide colloid and while uranium pseudo-colloid is easily removed during aging process, but thorium hydroxide colloid is not easily eliminated. And this shows new method that co-precipitate thorium hydroxide colloid and calcium hydroxide by adding the calcium oxide to remove radioactive materials from the sodium phosphate perfectly. If the concentrate of sodium phosphate solution is too high or addition of calcium oxide is too much, in the removal process of radioactive colloid the loss of sodium phosphate is increased or the quality of it is inferior as calcium includes in sodium phosphate solution. The solution is stirred under the conditions of stirring time of 10min, temperature of 80℃, sodium phosphate concentration of 40g/L and calcium hydroxide concentration in solution of 3g/L and deposited so as to remove radioactive materials perfectly. At that time the composition of sodium phosphate crystal was Na2HPO4∙7H2O-55.29%, Na3PO4∙12H2O-44.71% and its specific radioactivity was less than 30Bq/kg.