Au@Pd core shell nanoparticles were successfully synthesized via sequential reduction of Au(III) and Pd(II) ions with N,N-bis(2-hydroxyethyl)-2-aminoethanesulfonic acid (BES) at room temperature. Their size and morphology could be changed by varying the BES concentration and pH value of the reaction system. The Au@Pd core shell nanoparticles exhibited significantly higher catalytic activity for various Suzuki reactions than monometallic Pd or Au nanoparticles. The size-dependent catalytic activity was also observed, i.e., the Au@Pd core shell nanoparticles of <10 nm showed higher activity.