Uniform size and composition tuning of PtNi octahedra enclosed by 8 {1 1 1} facets were achieved by the control of specific crystal facet–surfactant bindings. In this chemical method, the optimal ratio of dioctyl amine to oleic acid is a key factor in the formation of PtNi octahedra with different alloying compositions, Pt:Ni = 18:6, 12:12, and 10:14. Careful electrochemical examinations show that the oxygen reduction reaction (ORR) mass activity and specific area activity of all PtNi octahedra outperform those of commercial Pt/C catalyst. The volcano-shaped trend of ORR enhancement proves that Pt12Ni12 octahedra have superior ORR activities. Further firm support is provided by the theoretical approach, which concludes that the adsorption energies of the H, O, O2, and OH species on a surface of Pt12Ni12(1 1 1) are much higher than those for Pt18Ni6(1 1 1) and Pt10Ni14(1 1 1) counterparts, whereas the molecularly adsorbed H2O product is calculated to be the lowest in adsorption energy on the Pt12Ni12(1 1 1) surface.