A green method was applied to prepare composites of multi-walled carbon nanotubes (MWNTs) decorated with silver nanoparticles (Ag-NPs). The MWNTs were functionalized by using mechanical ball milling technology in the presence of ammonium bicarbonate. The functionalized MWNTs were decorated with Ag-NPs by the traditional method of using a silver mirror reaction. Scanning electron microscopy (SEM) characterization showed that Ag-NPs distributed uniformly on the walls of MWNTs. The content and size of Ag-NPs could be controlled by adjusting the reducing time. The X-ray diffraction (XRD) patterns demonstrated that the Ag-NPs crystallized well. The resulting Ag/MWNT composites were used to prepare a water based nanofluid. The nanofluid showed higher thermal conductivity compared to that of a nanofluid containing MWNTs with functionalized surfaces. (C) 2012 Elsevier B.V. All rights reserved.