Studies of preparation condition and characteristics of AUC (ammonium uranyl carbonate) were carried out to optimize AUC process with different Precipitation methods. As results, four types of precipitates with different chemical compositions and morphologies were obtained from the reaction of (NH₄)₂CO₃ with UO₂(NO₃)₂ solution. A phase diagram has been made, and crystal structure and chemical composition of each phase have been characterized by using SEM, x-ray, IR and thermal analysis. It was found that ammonium uranyl carbonate, (NH₄)₄UO₂(CO₃)₃ with monoclinic crystal morphology could be synthesized when the mole ratio of (NH₄)₂CO₃/UO₂(NO₃)₂ in the solution was higher than 5.