Alloying　has　significantly　upgraded　the　oxygen　reduction　reaction　(ORR)　of　Pd-based　catalysts　through　regulating　the　thermodynamics　of　oxygenated　intermediates.　However,　the　unsatisfactory　activation　ability　of　Pd-based　alloys　toward　O2　molecules　limits　further　improvement　of　ORR　kinetics.　Herein,　the　precise　synthesis　of　nanosheet　assemblies　of　spin-polarized　PdCu-Fe3O4　in-plane　heterostructures　for　drastically　activating　O2　molecules　and　boosting　ORR　kinetics　is　reported.　It　is　demonstrated　that　the　deliberate-engineered　in-plane　heterostructures　not　only　tailor　the　d-band　center　of　Pd　sites　with　weakened　adsorption　of　oxygenated　intermediates　but　also　endow　electrophilic　Fe　sites　with　strong　ability　to　activate　O2　molecules,　which　make　PdCu-Fe3O4　in-plane　heterostructures　exhibit　the　highest　ORR　specific　activity　among　the　state-of-art　Pd-based　catalysts　so　far.　In　situ　electrochemical　spectroscopy　and　theoretical　investigations　reveal　a　tandem　catalytic　mechanism　on　PdCu-Fe3O4　&　horbar;Fe　sites　that　initially　activate　molecular　O2　and　generate　oxygenated　intermediates　being　transferred　to　Pd　sites　to　finish　the　subsequent　proton-coupled　electron　transfer　steps.