The surface treatment of silica particles with silane coupling agents was carried out by a dry treatment method in which the silane coupling agent was added to the particles and mixed using a mixer, and the reactivity with the particle surface was investigated. Both irregular-shaped and spherical amorphous silica particles were employed. gamma-glycidoxypropyl trimethoxysilane and gamma-aminopropyl triethoxysilane were used as silane coupling agents. The bonding to the particle surface was determined by a carbon analysis of unwashed and methanol-washed particles. This result shows that the amount of silane coupling agent added, the standing time after mixing, the heat treatment, the kind of the functional group of silane coupling agent, and the preparation method of silica particles all exerted an effect on the degree of bonding.