The synthesis of spherical hollow silica particles from sodium silicate solution with boric acid or urea as an additive was carried out by ultrasonic spray pyrolysis method. This work dealt with the effect of four parameters (boric acid, urea, feed rate of reactant, and reaction temperature) on particle size and standard deviation. As a result, the mean particle size and standard deviation decreased with increasing of all parameters except urea. In addition, silica particles with a variety of morphologies were prepared by varying the amount of boric acid and urea. Spherical particles with a smooth surface were prepared at the small concentration of boric acid (0.13-0.18 mole l(-1)) without urea. On the other hand, as the concentration increased (0.40-0.48 mole l(-1)), the Surface of particles became rough and porous condition. In addition, when a small amount of urea (0.07-0.15 mole l(-1)) was used, the spherical hollow silica particles were obtained. However, as the concentration increased (0.40-0.71 mole l(-1)), broken hollow silica particles with broad particle size distribution were synthesized. (C) 2004 Elsevier B.V. All rights reserved.