A study on calcination and reaction of AUC(ammonium uranyl carbonate, (NH₄)₄UO₂(CO₃)₃) has been carried out by using TG-DTA in N₂, air and H₂ atmospheres, respectively. As a results regardless of applied atmosphere AUC was calcined into amorphous UO₃, which was converted to α-U₃O₈ Via α-UO₃ in both H₂ and N₂ atmosphere, but directly into α-U₃O₈ in air atmosphere. Further reduction of U₃O₈ was only detectable in hydrogen atmosphere.