Current methods of colloidal nanosilica production are relatively energy-intensive and in some cases not environmentally friendly and therefore essential needs are felt to develop new low cost environmentally friendly methods. This study is devoted to the synthesis and characterization of colloidal silica nanoparticles prepared from silica fume using ultrasound. Colloidal nanosilica has been synthesized via dissolution-precipitation process followed by applying ultrasonic waves with the power and frequency of 30W and 20 kHz, respectively. The produced colloidal nanosilica was characterized via dynamic light scattering (DLS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectroscopy and measurements of its zeta potential and specific surface area. DLS results show that minimum particle size, average diameter and maximum particle size of the produced colloidal nanosilica decrease sharply from 28.21, 54.92 and 164.20 nm to 18.17, 38.72 and 141.80 nm, respectively, during the first 15 s of sonication. No significant changes have been observed in applying continued sonication up to 60 min. Measurements of zeta potential confirmed a relatively good stability of the produced colloidal nanosilica. (C) 2014 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.