en Research Paper Research Paper Abstract: Nano- size alumina particles have been synthesized by mechanical activation of a dry powder mixture of AlCl3 and CaO. Mechanical milling of the above raw materials with the conditions adopted in this study resulted in the formation of a mixture consisting of crystalline CaO and amorphous aluminum chlorides phases. There was no sign of chemical reaction occurring during milling stage as evidenced by x-ray diffraction studies. Subsequent heat treatment of the milled powder at 350ºC resulted in the occurrence of displacement reaction and the formation of Al2O3 particles within a water soluble CaCl2 matrix. The effect of higher temperature calcinations on the phase development in this powder mixture was followed by X-ray diffraction (XRD) analysis and scanning electron microscope ( SEM). Differential thermal analysis (DTA) was used to compare the thermal behavior between the milled and unmilled powders. Perhaps the most important result in this study was the observation of á-Al2O3 phase at a very low temperature of 500ºC. Mechanochemical synthesis; AlCl3; CaO;Al2O3; Nanoparticle; Heat treatment; Phase development 26 30 http://ijmse.iust.ac.ir/browse.php?a_code=A-10-3-47&slc_lang=en&sid=1 M. Ebrahimi-Basabi, 18003194753284600410 18003194753284600410 No J. Javadpour, javadpourj@iust.ac.ir 18003194753284600411 18003194753284600411 Yes H. Rezaie 18003194753284600412 18003194753284600412 No M. Goodarzi 18003194753284600413 18003194753284600413 No