||Palladium (Pd) is widely used in chemical industry for many applications such as hydrogenation, hydrocracking, decarboxylation etc. To use more efficiently, the Pd was often highly dispersed on various supports. It is well known that the catalytic activity and selectivity are strongly affected by particle size of catalysts and support properties. However, it is still difficult to obtain nano-sized Pd particles with uniform distribution. Herein, highly dispersed Pd nanoparticles depositied on carbon, silica, and alumina supports were prepared by chemical reduction. The CO-chemisorption results showed that the Pd/C, Pd/Al2O3, and Pd/SiO2 catalysts had Pd dispersion of 50%, 44%, and 13%, respectively. The catalytic activities of Pd supported catalysts were investigated in hydrogenation of maleic anhydride. The results demonstrated that the catalytic activity and selectivity were strongly dependent on the particle size of Pd in Pd supported catalysts.