The γ-alumina powder is a potential material for washcoat application. This paper shows the preparation of γ-alumina using ultrasonic technique in order to obtain nanocrystalline of γ-alumina powder. The 50 gr of the γ-alumina powders was treated in the 250 gr ethanol solution using ultrasonic technique within 10, 20, 30, and 60 minutes, respectively. The process of sonication was set at the fixed frequency (18.520 kHz) and current of 2 A. Crystallite size was evaluated by using X-ray diffraction (XRD) data based with Scherrer analysis and Williamson-Hall method. The microstructural characterization of the γ-alumina powders was observed using Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDX) analysis. The result shows that the optimum of nanocrystalline size of γ-alumina at ultrasonic process for 30 minutes are approximately 4.80 nm and 29.50 nm with Scherrer and Williamson-Hall calculation, respectively.