Rational synthesis of bi- or multi-metallic nanomaterials with both dendritic and porous features is appealing yet challenging. Herein, with the cubic Cu2O nanoparticles composed of ultrafine Cu2O nanocrystals as a self-template, a series of Pd-Cu nanocrystals with different morphologies (e.g., aggregates, porous nanodendrites, meshy nanochains and porous nanoboxes) are synthesized through simply regulating the molar ratio of the Pd precursor to the cubic Cu2O, indicating that the galvanic replacement and Kirkendall effect across the alloying process are well controlled. Among the as-developed various Pd-Cu nanocrystals, the porous nanodendrites with both dendritic and hollow features show superior electrocatalytic activity toward formic acid oxidation. Comprehensive characterizations including three-dimensional simulated reconstruction of a single particle and high-resolution transmission electron microscopy reveal that the surface steps, defects, three-dimensional architecture, and the electronic/strain effects between Cu and Pd are responsible for the outstanding catalytic activity and excellent stability of the Pd-Cu porous nanodendrites.