Transition metal oxides (TMOs) have attracted great attention as alternative oxygen evolution reaction (OER) electrocatalysts. While nanostructure engineering has been a widely used strategy to enhance the performance of TMOs, it is still not well-understood whether the reaction kinetics remain the same after shrinking the material size to nanometers. Herein, we report a modulated reaction pathway on SrNb0.1Co0.7Fe0.2O3−δ(SNCF) by shrinking its size to nanometers, leading to strongly enhanced OER activity. SNCF nanofibers with different diameters were synthesized by the electrospinning approach. In comparison to the bulk reference sample, more oxygen vacancies were observed on the surface of SNCF nanofibers. These oxygen vacancies lead to a decoupled proton and electron transfer on the surface of SNCF nanofibers, as revealed by the combination of the pH dependence study and H/D isotopic labeling experiments. The knowledge obtained in this work can be applied to guide the design of highly active catalysts for other reactions for energy and environmental devices.