A highly effective polyelectrolyte functionalization of multi-walled carbon nanotubes (MWCNTs) by poly(diallyldimethylammonium chloride) (PDDA-MWCNTs) was employed for low temperature fuel cell applications. PDDA-MWCNTs were employed as support materials for the in situ deposition and formation of platinum nanoparticles, via the self-assembly between the negative Pt precursor and positively charged functional groups of PDDA-functionalized MWCNTs. The effect of the functionalization on the deposition and distribution of Pt nanoparticles was investigated in detail. Compared with MWCNTs functionalized by conventional acid-oxidation treatment (AO-MWCNTs), the PDDA-functionalized MWCNTs cause no structural damage on MWCNTs and provide high density and homogeneous surface functional groups for the anchoring Pt nanoparticles. Pt nanoparticles with an average particle size of 1.8 ± 0.4 nm and loading as high as 60 wt% were realized on PDDA-MWCNTs supports. The Pt/PDDA-MWCNTs electrocatalysts show significantly higher electrochemically active surface area and higher electro-catalytic activity for methanol oxidation than that of Pt/AO-MWCNTs and E-TEK Pt/C electrocatalysts.