The relationship between coverage of hydrophobic groups on modified glass bead surfaces and macroscopic properties such as wettability or dispersion ability has been demonstrated. The glass bead surfaces modified by trimethylsilyl (TMS) groups were obtained by the chemical reaction between trimethylchlorosilane molecules and surface silanols. The TMS coverage was estimated from carbon content by combustion gas analysis, using IR spectroscopy and the specific surface area determined by krypton adsorption. The macroscopic wettability of the samples for water was examined by floating tests and measuring contact angles. The results are summarized as follows. (1) The surface determination, employed in this study, enables one to measure small amounts of TMS groups on small-surface-area samples. (2) It was recognized from the results of the floating test and the contact angle measurement that macroscopic wettability changed markedly at the TMS coverage of about 50% of monolayer coverage. (3) The simulation and model of wetting can account for the wettability of a modified glass-bead particle surface. It was demonstrated that the wettability of material had a close relation to the bulkiness of modifier and the surface coverage on the molecular level.