With an accurate control oldie dispersity and size of the palladium nanoparticles (Pd NPs), carbon spheres/Pd NPs composite was prepared without any extra reducing agents. In order to fully understand the formation mechanism and find out the best condition for the fabrication of carbon/Pd composite spheres, the effects of temperature, reaction time, pH value, and the weight ratio of PdCl2 to carbon spheres on the morphology of the final products were investigated. A superior product with small (d = 7.66 nm, sigma = 1.94 nm), homogeneously distributed Pd crystals was obtained at pH 7 and a reaction temperature of 70 degrees C in ethanol. The Pd NPs decorated carbon sphere was used as support for electroactive polyaniline (PANI) in our work because it could enhance their sensing properties which were afforded by catalytic Pd NPs and hydrophilic carbon spheres. The sensor based on carbon/Pd/PANI exhibited a high sensitivity of 656.0693 mA M-1 cm(-2) and a detection limit of 5.48 mu M toward the reduction of H2O2. In addition, the carbon/Pd/PANI sensor also showed good selectivity between H2O2 and ascorbic acid.