Composite ZnO/SiO2 nanoparticles were made by flame spray pyrolysis (FSP). Characteristics of the product powder and its crystallization behavior on post-calcination were evaluated. Polyhedral aggregates of nano-sized primary particles consisting of ZnO nano-crystals 1 - 3 nm in size and amorphous SiO2 were obtained by FSP. A short residence time in the flame can result in the co-existence of the ZnO and SiO2 clusters without substitution or reaction hindering each other's grain growth. There was almost no change in the XRD pattern by calcination at 600degreesC for 2 h, suggesting a high thermal stability of the ZnO nanocrystals in the composite particles. A pure alpha-willemite phase was obtained at 900degreesC. At this calcination temperature, d(C) and d(BET) Of the powder were 63 and 44 nm, respectively. The nano-composite structure of the FSP-made particles can suppress crystalline growth of ZnO during calcination to maintain a high reactivity of ZnO with SiO2,, obtaining pure a-willemite with high specific surface area at low calcination temperatures.