Sound-assisted fluidization of two kinds of SiO2 nanoparticles (having primary sizes of 5-10 nm) is investigated in this study. The two kinds of nanoparticles are SiO2 without any surface modification (NSM-SiO2) and SiO2 modified with an organic compound (SM-SiO2). The introduction of a 99.8-103.4 dB and 50 Hz acoustic field reduces the superficial minimum fluidization gas velocity, Umf,super, significantly for the two kinds of nanoparticles, and when the sound pressure level increases, the values of Umf,super decrease gradually. With the agitation of a 100 dB acoustic field, the SM-SiO2 nanoparticles could fluidize smoothly similar to Geldart-A particles over the frequency range of 40-60 Hz. NSM-SiO2 failed to fluidize so smoothly, but significant improvement was observed over such a frequency range. Different fluidization behavior, different bed expansion, and agglomerating behavior were also observed for the two kinds of nanoparticles, which indicate that the surface properties of nanoparticles have significant influences on their fluidization behaviors.