The precursors of Co3O4 and Ag/Co3O4 composites with different Ag contents were synthesized with assistance of (NH4)2CO3 via a facile hydrothermal process. The final samples were fabricated by calcining each precursor at 400 °C according to TG experiment. The as-prepared samples were identified and characterized by thermogravimetric analysis, X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, and field emission scanning electron microscopy, respectively. The results showed that the morphology of Co3O4 and Ag/Co3O4 composites were sheet-like. Ag nanoparticles were dispersed well in the nanosheets. The samples were used as electrocatalysts modified directly on a glassy carbon electrode for p-nitrophenol reduction in a basic solution. The results showed that p-nitrophenol could be reduced at a large peak current but a higher peak potential with Co3O4, at lower potentials with Ag/Co3O4 composites. Ag/Co3O4 composite with 3 % Ag showed more efficiently electrocatalytic activity than other composites. The present method suggested the potential application of Ag/Co3O4 composites in electrocatalysis.