The quality of dispersion of nanoparticles, which agglomerate strongly, greatly influences the processing process and the quality of final products such as sintered bodies. Therefore, the technology of nanoparticle dispersion is considered as an important element in industry. In the present work, aqueous suspensions of nanoparticles were produced by a high pressure wet-type jet mill, using SiO2 (primary particle diameter: about 20 nm) and Al2O3 (primary particle diameter: about 13 nm) as dispersed particles. The effects of the operating conditions on the mean size of particles, and the relation of the mean particle size to aqueous suspension viscosity were examined. The volumetric mean diameter (d(v)) decreased with increasing processing pressure (P), number of passages (N) and concentration of particles (C), but did not decrease to the primary particle size. The agglomerated particle size was 5.9-7.4 and 6.5-8.1 times the primary particle size for SiO2 and Al2O3, respectively. Therefore, dispersion became increasingly difficult as primary particle size decreased. For all the present experimental conditions, the relationship between d(v) and the energy dissipation rate (epsilon) could be expressed by a straight line with a slope of -0.03 on logarithmic paper. Aqueous suspensions of both types of particles exhibited plastic behavior at N=O, and pseudo-plastic behavior at Ngreater than or equal to1. Furthermore, the pseudo-plasticity decreased with increasing N, and at Nigreater than or equal to10 10 the aqueous suspensions apparently exhibited Newtonian behavior.