Three types of SiO2 ultrafine particles were used to investigate the fluidization behavior in a fluidized bed with sound excitation. It has been shown that agglomerate size tends to reduce with an increase in sound pressure level. At a given sound pressure level, there exists a critical sound frequency (f(c)) Agglomerate size is decreased with increasing sound frequency as sound frequency less than f(c). Whereas, agglomerate size intends to grow with an increase in sound frequency as sound frequency exceeds f(c). A mathematic model to predict the agglomerate size has been developed based on energy balance between the agglomerate collision energy, the energy arising from sound wave, and cohesive energy. The model can predict qualitatively the effects of sound frequency and sound pressure level on agglomerate size. (c) 2006 Elsevier B.V. All rights reserved.