To investigate the electrocatalytic performance of PtRu nanoparticles supported on multi-walled carbon nanotubes (MWCNTs) with different lengths and diameters, X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and cyclic voltmmetry experiments were conducted. It is demonstrated that the length and diameter of MWCNTs play an important role in the electrocatalytic performance of PtRu catalysts. The co-existence of amorphous carbon impurities on the MWCNT10-2 support lowered the accessible surface area of the PtRu nanoparticles, hampered the dispersion of the PtRu nanoparticles, and induced the formation of a low degree of PtRu alloy, thus lowered the electrocatalytic performance of the PtRu/MWCNT10-2 catalyst for methanol oxidation. The highest mass-specific activity of PtRu/MWCNT3050-2 results from a highly accessible PtRu surface and a good dispersion of PtRu particles. Our experimental results also demonstrate that the tube length of MWCNT samples has little effect of the Surface area specific activity of the PtRu/MWCNT catalyst, whereas the PtRu nanoparticles supported on the MWCNT samples with large tube diameter tends to exhibit a higher surface area specific activity for methanol oxidation reaction. This result is suggested to be the combined effects of a high degree of PtRu alloying and the high electronic conductivity of these MWCNT samples. (C) 2008 Elsevier Ltd. All rights reserved.