The thermal decomposition behaviour of UO3 gel microspheres has been studied. Thermal analysis involved DTA, TG, DTG and X-ray examination. The effects of washing the UO3 gel microspheres with hot water and with ammonia solution were investigated, as were the effects of soaking the microspheres in ammonia solution before or after washing on the crystallite size. The results indicate that the thermal decomposition of UO3 gel microspheres includes five steps: the first two for dehydration, the third for ammonia removal, the fourth for ammonia oxidation and the last step for UO3 recrystallization.