It is very difficult to handle nanoparticles in dry process, thus, nanoparticle slurries are widely used for material fabrication processes. Since the particle dispersion and aggregation state has a great influence on the quality of various products fabricated from nanoparticles. However, the characterization techniques for the particle dispersion state of dense nanoparticle slurries have not been established, yet. Therefore, in this study, a novel evaluation technique, in which the osmotic pressure of nanoparticle slurry is measured, has been developed in order to characterize the dispersion state of nanoparticles in the slurry. It was shown that the osmotic pressure of nanoparticle slurry increased with a decrease in the median diameter of the slurry, indicating that the well dispersed slurry had larger osmotic pressure and we can evaluate the dispersion state of nanoparticles through the osmotic pressure measurement. It was also demonstrated that the osmotic pressure measurement could be useful for dense and/or less transparent slurries although the conventional particle size measurement, dynamic light scattering, was not applicable.