Waterborne raspberry-like PMMA/SiO2 nanocomposite particles were prepared via a free radical copolymerization of methyl methacrylate (MMA) with 1-vinylimidazole (1-VID) in the presence of ultrafine aqueous silica sols. The strong acid-base interaction between hydroxyl groups (acidic) of silica surfaces and amino groups (basic) of 1-VID was strong enough for promoting the formation of long-stable PMMA/SiO2 nanocomposite particles when 10 mol % or more 1-VID as auxiliary monomer was used. The average particle sizes and the silica contents of the nanocomposite particles could range from 120 to 350 nm and 5 to 47%, respectively, depending upon reaction conditions. The influence of synthetic conditions, for instance, initial silica charge, reaction temperature, pH value, and 1-VID charge, on the particle properties was studied in detail. Stable nanocomposite particles could only be obtained at base conditions, and the silica content in the nanocomposite particles reached the maximum when pH value was 8.0. TEM and SEM indicated a raspberry-like morphology of the obtained nanocomposite particles. Zeta potential measurements confirmed that part of the 1-VID was located on the surface of the nanocomposite particles, which was consistent with the raspberry-like morphology. A possible formation mechanism of nanocomposite particles was proposed.