Au-Pd bimetallic nanocrystals with well-defined dendritic morphology, uniform size, and homogeneous alloy structure were synthesized in an aqueous solution by using ascorbic acid as the reductant and triblock copolymer P123 as the stabilizing agent. In this synthesis, ascorbic acid played a key role in directing the nanodendritic morphology while the presence of Pd was mandatory for the formation of well-defined nanodendrites. Without Pd, only faceted nanoparticles of gold were formed under the same reaction conditions. Other reaction variables such as the addition sequence and the dropping rate of the metal precursors, the type and concentration of the stabilizing agents, and the reaction time and temperature were all found to affect the size uniformity and morphology perfectness to some extent. The Au/Pd atomic ratio could be tuned in a wide range without deteriorating the nanodendritic morphology. The as-prepared Au-Pd nanodendrites exhibited excellent catalytic activities toward electroxidation of methanol and reduction of 2-nitrophenol, and the catalytic performances could be effectively tuned by the Au/Pd ratio.