Pd-SnO2 heteroaggregate nanocatalysts were synthesized by in-situ transformation of PdSn bimetallic nano particles on alumina. The PdSn bimetallic nanoparticles were prepared by co-reduction of Pd and Sn precursors. The obtained PdSn nanoparticles possibly have a core-shell like structure with Pd cores and Sn-rich shells due to the fast reduction of Pd precursors. The Pd-SnO2/Al2O3 heteroaggreagte nanocatalysts were synthesized by calcination of PdSn/Al2O3 in air followed by H-2 reduction. Various characterization techniques, such as HRTEM, XRD and XPS, were used to characterize the PdSn and Pd-SnO2. The Pd-3-(SnO2)(1)/Al2O3 heteroaggreagte nanocatalysts with a ratio of Pd/Sn of 3/1 illustrated enhanced catalytic performances forp-nitroacetophenone and p-nitrobenzaldehyde hydrogenations relative to monometallic Pd nanocatalysts. Theoretical calculations using density functional theory suggested that Pd-SnO2 facilitates the adsorption of reactants in a higher degree than that of products, thus enhancing the catalytic activity even with partial coverage of Pd surfaces with SnO2. The blockage effect of SnO2 on Pd surfaces is speculated to enhance the catalytic selectivity for p-nitroacetophenone and p-nitrobenzaldehyde hydrogenations over Pd-SnO2 heteroaggregate nanocatalysts.