This study investigated the thermal decomposition property of the sludge contained uranium in lagoons of uranium conversion plant. The lagoon sludges consist mainly of nitrate salts of ammonia, sodium, and calcium. As result of analysing TG/DTA, ammonium nitrate was decomposed around 300 ℃, calcium nitrate, calcium hydroxide, and sodium nitrate around 600 ℃. The amounts of residues after decomposing completely the lagoon sludges at 800 ℃ was similar to the results from TG analysis. The lagoon sludges could be removed over 80 wt% as radioactive waste by thermal treatment.