With the growing interest in nanoscale materials, the surface coating on nanoscale is a demanding technology. In the present work, the rapid expansion of supercritical suspensions in impinging-stream reactors is proposed for the coating of ultrafine particles with the coating layer on nanoscale. Ultrafine particles with the average particle size of 1 mum and 20 nm were used as the core particles. The ultrafine particles were well dispersed in the supercritical solutions of coating materials to form supercritical suspensions. By means of the rapid expansion of the supercritical suspensions from two capillary nozzles, the coating process was carried out in an impinging-stream reactor. The coating result was examined by the SEM observation. A uniform coating of ultrafine particles was achieved in this process. The effect of impinging distance on the coating rate was also investigated. (C) 2003 Elsevier B.V. All rights reserved.