The molecular dynamics (MD) method and IR spectroscopy have been applied to investigate the conformations of several normal alkoxy groups grafted on a silica surface. The structures of octhyl (C8), dodecyl (C12) and octadecyl (C18) alkoxy groups at about a maximum loading value were focused on. The conclusions of the study were as follows. (i) The difference in the maximum lending values between Cs and C18 alkoxy groups was estimated to be 20% at most, which was smaller than that expected from the bulkiness of the groups. (ii) The results of the MD simulation are nearly in accordance with those obtained by IR measurements. The fraction of that structure in the total segments of the groups introduced is constant or decreases slightly depending on their lengths. (iii) The length effect of the groups on the conformation was assumed by MD simulation. These estimations are not obtained by the experimental method. It is suggested that MD simulation is very useful for surface design and prediction of surface properties. (iv) The results of MD simulation and IR measurement demonstrate that at a higher degree of modification of a few segments near the modified sites is rigid and the terminal part of the alkoxy group is flexible.