Aluminum oxide, Al2O3, nanoparticles were synthesized from alcoholic solution of Aluminum Chloride, AlCl3, and ammonia via sol-gel process. Molar ratio of NH3: AlCl3 has been changed to control the size of the resultant nanoparticles. The structures of Al2O3 nanoparticles were investigated and confirmed using Fourier Transform Infrared spectroscopy (FT-IR) and X-ray diffraction (XRD), and their morphology was studied via high resolution transmission electron microscopy (HR-TEM). The HR-TEM images showed that the Al2O3 nanoparticles have spherical shape with a particle size ranging from 7 nm to 50 nm depending on the precursor’s concentrations. It is found that higher ammonia and water concentration resulted in increasing the particle size of the Al2O3 nanoparticles. Micro particles of Al2O3 were obtained when the way of the precursors’ addition has been altered; adding the ammonia to the alcoholic solution of the AlCl3. The proposed mechanism behind the nanoparticles preparation as well as the dependence of their size on the selected factors has been discussed.