The present paper describes a study on particle size distribution of alumina powders obtained from thermal decomposition of ammonium dawsonite. The ammonium dawsonite, an ammonium aluminum carbonate hydroxide, was synthesized from solutions of ammonium alum and ammonium hydrogen carbonate, and thermally decomposed at different temperatures. According to the results of the particle size distribution, the values obtained from bi-modal curves were analyzed for determination of the average grain-size in each model.