Ultrafine Pd nanoparticles homogeneously deposited on citrate-functionalized graphene oxide (PdNPs/Cit-GO) were synthesized by a simple method with the reduction of ascorbic acid. The average size of these PdNPs was 4.67 nm, and they dispersed on the GO surface with high density. Significantly, the as-prepared PdNPs/Cit-GO expresses a high catalytic activity in the reduction of 4-nitrophenol (4-NP). The synergistic effect of the PdNPs/Cit-GO in the catalysis reaction was also demonstrated.