The focus of this work is the determination of the size of aggregates and the effect of superficial gas velocity on particle aggregation in a nano-particle fluidized bed. Degussa Aerosil R974 powder, with a primary particle size of 12nm, was fluidized using nitrogen in a glass vessel of 46mm square cross-section. The superficial gas velocities at which laser-based planar imaging experiments were performed ranged from about 4-8 times the minimum fluidization velocity. Aggregates of nano-sized particles were recorded using in situ planar laser-sheet illumination with a high-resolution digital CCD camera. The aggregates were found to have a wide size distribution and contain a significant component of very small sizes. The mean size of the aggregates was found to decrease with increasing superficial gas velocity, while the mean sphericity of the aggregates was found to be insensitive to changes in the superficial gas velocity. The measurements based on planar laser-sheet imaging are compared with the results of previous studies and to predictions of existing models. (c) 2006 Elsevier Ltd. All rights reserved.