Pd-Ir mesocrystals with rough surface and highly branched structure were synthesized through a facile co-reduction method. The morphology evolution shows the mesocrystals were formed by oriented attachment of nanoparticles, and abundant defects sites as well as synergetic effect between building units were formed during mesocrystals oriented attachment process. Therefore, the supported Pd-Ir mesocrystals show a significant enhancement in catalytic performance in hydrogenation of 2-ethylanthraquinone compared with conventional Pd-Ir nanoparticles. The high activity can be ascribed to high densities of defect sites in Pd-Ir MCs catalyst, which facilitated H-2 activation. In addition, the geometric effect of Pd-Ir MCs catalyst was responsible for inhibiting further hydrogenation of eAQH(2) to undesired consecutive byproducts.