Ammonium diuranate (ADU) and its heating product uranium oxides (UO₃ + U₃O₈) are important intermediates for nuclear fuel production. The microstructure and particle size distribution (PSD) of ADU powder produced at various stages of precipitation have been measured to study the particle growth of ADU. The change of morphology of ADU and UO₃ + U₃O₈ has also been studied when gaseous ammonia is added instead of aqueous ammonia solution and when a seeding agent is added to the uranyl nitrate solution before precipitate formation. It has been observed that the ADU particles form basically agglomerates of submicron platelets. The microstructure of ADU was retained in UO₃ + U₃O₈ even after calcination.