This study is mainly concerned with the electrodeposition of palladium on the copper alloy lead frame used for semiconductor assembly process. The role and effect of additives on palladium electrodeposition were studied by using various electrochemical methods. Ortho-formylbenzenesulfonic acid as a primary additive was used in palladium plating on the lead frame in this study. The electrochemical characteristics of electrode reaction were measured by the hanging mercury drop electrode for electrochemical system and the qualities of the plated surface of lead frames were also examined. The additive agent in Pd solution could have been classified as the grain refiner. It acted as the electroactive species, which increased the polarization and decreased the roughness, by adsorption on the electrode in palladium pre-plated process. The reduction of palladium ion was identified to be an irreversible reaction and the diffusion coefficient of palladium ion and the reaction rate constant were obtained from chronopotentiometry experiment.