Oxygen reduction reaction (ORR) is becoming increasingly important with the development of fuel cells and metal-air batteries. Manganese oxides have been one of the focuses of recent research for Pt-alternative ORR catalysts. However, the structure-activity relationships of manganese oxides have not been well studied or understood. In the present work, we report a new finding that there is a strong dependence of the ORR activity of Mn3O4 on its morphology. By adopting different solvents in the wet-chemical synthesis, we are able to tailor the morphology of Mn3O4 from nanoparticles (NP-L, 12.5nm and NP-S, 5.95nm) to nanorods (NR, exposure of Mn3O4 (101)) and nanoflake (NF, exposure of Mn3O4 (001)). Surprisingly, surface-specific activity of NF toward the ORR was found to be one order of magnitude higher than NP-L. The morphology-activity relationships of Mn3O4 were further studied through a combination of electrochemical experiments and density functional theory (DFT) calculations. It was discovered that the formation of *OOH, concomitant with the first electron transfer, is the potential determining step, which is thermo-dynamically more facile on Mn3O4 (001) than (101) plane. The underlying mechanism could be ascribed to the strong interaction between O2 and Mn3O4 (001) surface as indicated by the DFT calculations. The study enlarges our understanding of Mn3O4 catalysis and provides clues for rational design of highly efficient transitional metal oxide electrocatalysts for the ORR.