Surface-modified silica gel was produced to improve the surface affinity to an oily medicine, phytonadione (VK 1). The effect of the degree of surface modification of the silica gel on the drug release behavior from the silica porous matrix was investigated. The silica gel was surface modified using the silan coupling agent, 3-methacryloxypropyl-trimethoxysilane (C 7), octadecyltriethoxysilane (C 18), or 3,3,3-trifluoropropyltrimethoxysilane (F 3). A mixture of VK 1 solution and surface-modified silica gel was evaporated under reduced pressures at room temperature, then the resulting powder was dried in vacuo. The degree of surface modification was evaluated from elementary analysis. The dissolution profiles of the samples were investigated in Japanese Pharmacopoeia XII, 1st fluid buffer (pH 1.2, 37+0.5^oC) containing 1.5% sodium lauryl sulfate. The FT-IR spectra of VK 1-loaded surface modified-silica gels suggested that the amount of hydrogen-bonded VK 1 with the silanol group on the gel surface decreased with increasing hydrophobicity of the silica gel. Since the modified group was rotating on the silica gel surface and inhibited the adsorption of VK 1 to the surface, the attractive molecular interaction between VK 1 and the silica gel surface might decrease with increasing length of the modified functional group. However, the characteristics of the affinity of VK 1 to the functional groups significantly differed among the groups. The VK 1 release from the modified silica gels was initially rapid, slowed markedly after 1 h, and continued for more than 24 h. The amount of VK 1 released from the modified surface silica gels by C 7, C 18, or F 3 increased with increasing density of the surface modification group. The mean drug release moment (MDT) decreased with an increase in surface modified group density.