The wettability and surface structure of precipitated silica surface loaded trimethylsilyl groups were investigated. By measuring water and nitrogen adsorptions, it was found that the nontreated sample had micropores. The pores with a diameter less than 0.8 nm were not modified, while the pores with a diameter from 0.8 to 0.9 nm were modified by trimethylsilyl groups and closed by them. From these results, it was assumed that the former pores would become sites for water vapor as surface silanol groups after modification and the latter pores would be independent of water adsorption. The character of the silica bare surface disappeared above a surface conversion rate of 33%. These results are deduced from neopentane adsorption experiments. Furthermore, the water adsorption capacity near saturated pressure, as an index of micro-wettability, did not reach the theoretical monolayer capacity above the same conversion rate (33%). It was also indicated that the preferential dispersion property as an index of macro-wettability drastically changed at a conversion rate of about 33%. From these results, it was demonstrated that the wettability is closely related to the surface structure interfering with the adsorption of water.