We fabricated RuO2/carbon nanotube (CNT) composites by coating the CNTs with an amorphous RuOx layer using metallorganic chemical vapor deposition followed by postannealing at a relatively low temperature of 350degreesC. It was found that the kind of gas used during the postannealing process should affect the nanostructure of the RuO2/CNT composites quite significantly. Starting with a thin RuOx, coating layer, we could obtain relatively uniform-sized spherical and elliptical RuO2 nanoparticles by using Ar and O-2 gas, respectively, during the postannealing. The growth behaviors of the RuO2 nanoparticles could be explained in terms of surface self-diffusion. The surface diffusion process can provides a way to fabricate RuO2/CNT nanocomposites with a controllable geometrical shape and size, whose process can be used for diverse applications, such as high-performance supercapacitor electrodes, electrocatalysts, gas sensors, and new electrodevices. (C) 2004 The Electrochemical Society. All rights reserved.