Fly ashes are the waste product following the combustion of coal. Disposal of fly ashes is a serious environmental problem; therefore, finding a new utilization of fly ashes is highly demanded. Carbon nano-structures have been widely studied as key materials in a wide range of applications, including sensing, saturable absorber, and so on, because of its optical and electromagnetic absorption property. Adding the property of carbon nanotubes (CNTs) to fly ashes expands applications of fly ashes. In this paper, we present the fabrication of multi-walled carbon nanotubes (MWCNTs) on fly ashes by thermal chemical vapor deposition (CVD) process, which result in simple, low-cost, large-amount production of MWCNTs. Copyright © 2009 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.