We report herein an in situ electrochemical scanning tunneling microscopy (ECSTM) investigation of iron-phthalocyanine (FePc)-catalyzed oxygen reduction reaction (ORR). A highly ordered FePc adlayer is revealed on a Au(111) electrode. The center ions in the FePc adlayer show uniform high contrast in an oxygen-saturated electrolyte, which is attributed to the formation of an FePc-O2 complex. In situ STM results reveal the sharp contrast change upon shifting the electrode potential to trigger the ORR. Theoretical simulation has supplied further evidence for the contrast difference of the adsorbed FePc species.