Barium zirconate fine powders of pure cubic phase were readily prepared from the reactions between BaCl22H2O and ZrOCl28H2O under ammonothermal conditions at a very low temperature of 130oC as the lowest. KOH concentration was important in determining phase formation and particle morphology. Reaction temperature and time showed influences on the evolution of particle morphology and aggregation formation, respectively. Sizes of the primary particles critically depended on the BaII:ZrIV mole ratio. Phase formation mechanism is suggested.