The Korea Atomic Energy Research Institute (KAERI) has begun a decommissioning program for a uranium conversion plant. A treatment process for the sludge waste, which was generated during the operation of the plant and stored in lagoons, was proposed based on the results of an analysis of the sludge characteristics and the thermal decomposition properties. The thermal decomposition rate was observed by a load cell at a given heating rate. The solid waste…s stabilization properties were analyzed by TG/DTA and XRD. The nitrate salts in the sludge were decomposed in two steps; the first decomposition occurred at ca. 300 ℃ and the second at over 600 ℃. The low-temperature decomposition was due to ammonium nitrate at 300 ℃; the high-temperature decomposition was due to those of sodium and calcium nitrate and calcium carbonate at 900 ℃. Alumina should be added for stabilization of the sodium oxide, which arose from decomposition of the sodium nitrate and reacted readily with water. The residual solid waste consisted mainly of Na2O；2UO3, calcium oxide, calcium hydroxide, and Na2O；Al2O3; these materials are stable compounds for storage. As a result, the volume of the sludge waste could be decreased by over 75 % when using this sludge treatment process.