Detailed experimental and theoretical studies have been performed to investigate the influence of the diameter of multi-walled carboxy-functionalized carbon nanotubes (CFCNTs) on their ability to reduce PdCl₄²⁻ salt to Pd nanoparticles on their surface at room temperature. The obtained results (inductively-coupled plasmaspectrometry and cyclic voltammetry) show that the reduction ability of CFCNTs with 5 nm diameter (CFCNT5) is stronger than that of CFCNTs with 15 nm diameter (CFCNT15). Density Functional Theory (DFT) calculations suggest that a more negative charge distribution exists on CFCNT5, which makes it a better electron donor to PdCl₄²⁻.