Stable water based nanofluids containing multi-walled carbon nanotubes (MWNTs) were prepared using cationic gemini surfactant as stabilizer. Zeta potential measurements and Fourier transformation infrared spectra were employed to study the absorption mechanisms of the surfactants on the MWNT surfaces. The stability of the nanofluids was obtained using UV-vis absorption method. Results of thermal conductivity indicate that higher concentration of cationic gemini surfactant is a negative factor in improving the thermal conductivity of nanofluids. Increase of spacer chain length of cationic gemini surfactant gives rise to the sediment of MWNTs in the nanofluids, resulting in decrease of thermal conductivity enhancement of MWNT nanofluids. Mechanical ball-milling technology would be a new method to pretreat the pristine MWNTs. and using this method nanofluids with optimized thermal properties can be obtained. (C) 2010 Elsevier B.V. All rights reserved.