Particle monolayer formation at the air-water interface by polymer-grafted colloidal silica was investigated. Methyl methacrylate (MMA) was polymerized from initiative bromide groups at colloidal silica surface by atom transfer radical polymerization. We obtained polymer-grafted silica particle (SiO2-PMMA) with relative narrow polydispersity of PMMA. For the polymer-grafted particle with high graft density, particle monolayer formation was confirmed by pi-A isotherm measurement and SEM observation. Interparticle distance was controllable by surface pressure. Furthermore, grafted polymer chains were suggested to be fairly extended at the air-water interface. However, for the polymer-grafted particle with low graft density, monolayer structure on substrate showed aggregation and voids. (c) 2006 Wiley Periodicals, Inc.