The nanoparticles (NPs) of Ni with different sizes endows its distinctive physical and chemical properties, which represents a typical strategy for the development of high-performance catalysts. However, the size effect of metallic Ni-NPs on electrocatalytic performance remains ambiguous. Herein, the Ni-NPs with different sizes supported on nitrogen doped carbon (NC) has been synthesized by controlling the pyrolysis temperature, leading to the synthesis of Ni@NC-500 (8.3 nm), Ni@NC-280 (1.9 nm) and Ni@NC-200 (1.0 nm). The electrooxidation of benzyl alcohol (BA) over these nanocatalysts shows the yield of benzoic acid was 99%, 82%, 55% on Ni@NC-280, Ni@NC-200 and Ni@NC-500, respectively. The experimental and theoretical simulation demonstrate that the difference in the adsorption strength of reactant molecules by Ni-NPs is responsible for their different performance, where the Ni@NC-280 exhibits an optimal adsorption configuration between Ni@NC-280 electrode and BA. This work provides a new angle for designing and synthesizing efficient electrocatalysts, which may be extended to the exploration of various promising electrocatalytic systems.