Spherical silica particles that are able to assemble at a phase boundary of a dual-phase mixture of water and an immiscible organic solvent were prepared by a partial modification of their surface hydroxyl groups with an alkylsilylation agent. Scanning electron microscopic observation of these particles in which their remaining surface hydroxyl groups had been selectively modified with colloidal gold particles revealed that each particle has an asymmetric surface structure: one side of the surface is hydrophilic and the other is hydrophobic. We found that these particles could form a micellar structure in water in the presence of an organic solution of a toluene/polystyrene mixture. The micellar structure was evidenced by formation of golf-ball-like polystyrene particles with dimples imprinting morphologies of the hydrophobic part of modified silica particles.