In order to elucidate the stability of a silicon oxide layer on zoo, the dynamic behavior of SiO- and SiO2-layer-coated ZnO ultrafine particle surfaces were directly observed by heating in a transmission electron microscope up to 700degreesC. In the case of SiO-coated ZnO ultrafine particles, ZnO nanocrystallites with the size of 2 nm grew in the SiO layer at 500degreesC. In the case of SiO-coated ZnO ultrafine particles, beta-cristobalite with the size of 10nm order was grown at 600degreesC. Zn2SiO4 particles were grown by coalescence between beta-cristobalite and ZnO at 700degreesC. By comparing the SiO and SiO, layers on ZnO ultrafine particles, it was found that the behaviors of the two silicon oxide layers are clearly different. (C) 2003 Elsevier B.V. All rights reserved.