Palladium nanoparticles were prepared by adding a small quantity of hexadecyl trimethyl ammonium bromide (CTAB) into the palladium (Ⅱ) chloride aqueous solution using ultrasonics. The samples were characterized by XRD, TEM, selected area electron diffraction (SAED), HRTEM and low temperature nitrogen adsorption-desorption. The effect of CTAB amount on the growth morphology of palladium nanoparticles was also investigated. The electrocatalytic properties of the nano-Pd modified glassy carbon (Pd/GC) electrode for formaldehyde oxidation were investigated by cyclic voltammetry. The results indicate that the size and the morphology of the nano-Pd particles can be controlled by adjusting the mole ratios of CTAB to PdCl2. The hexagonal nano-Pd particles with a narrowly distributed size of 8 nm are obtained when the mole ratio of CTAB to PdCl2 is 1:1 and the ultrasonic reaction time is 50 min. The nanoparticles obtained by CTAB addition possess larger BET surface area compared to that without surfactant by 10 m2/g, thus showing the excellent electrocatalytic activity for the formaldehyde oxidation.