The direct ethanol fuel cells in an alkaline medium have a broad vision of applications because of their large energy density, reasonable power density, and environmentally friendly features. Herein, we present a facile one-step method to synthesize PdAg nanosheet assemblies (NSAs) in a mixed solution of N,N-dimethylformamide and water with the addition of molybdenum hexacarbonyl and cetyltrimethylammonium bromide. Pure Pd NSA shows an irregular shape while PdAg NSAs gradually undergo a process from solid assembly to a hollow structure with the Pd/Ag molar ratio changing from 3:1 to 2:1 to 1:1. The formation of alloy nanosheets in the assemblies combined with the introduction of Ag in the Pd catalyst enhances the catalytic activity toward ethanol electrooxidation from 1524 mA mg-1 of pure Pd NSA to 1866 mA mg-1 of PdAg NSA with a Pd/Ag molar ratio of 2:1. On the basis of the experimental data, compared with pure Pd structures, both the nature of a thin nanosheet of PdAg NSAs and the structural changes in the alloy assemblies play key roles in determining the electrocatalytic activity of these Pd-based catalysts.