A versatile approach to fabricate monodisperse poly[styrene-co-(divinyl benzene)] (PS-co-DVB) microcapsules that contain a single gold nanoparticle (AuNP) has been demonstrated. Using the PS-co-DVB microcapsule as a microreactor, aqueous HAuCl4 and NaBH4 solutions are subsequently infiltrated. The size of the resulting AuNP inside of the PS-co-DVB microcapsules is easily tunable by controlling the repeate infiltration cycles of aqueous HAuCl4 and NaBH4. PS-co-DVB microcapsules that contain a single silver and palladium nanoparticle are also obtained by following a similar protocol.