For the first time, this work presents Au@Ag core-shell nanoparticles (NPs) immobilized on a metal- organic framework (MOF) by a sequential deposition- reduction method. The small-size Au@Ag NPs reveal the restriction effects of the pore/surface structure in the MOF. The modulation of the Au/Ag ratio can tune the composition and a reversed Au/Ag deposition sequence changes the structure of Au-Ag NPs, while a posttreatment process transforms the core-shell NPs to a AuAg alloy. Catalytic studies show a strong bimetallic synergistic effect of core shell structured Au@Ag NPs, which have much higher catalytic activities than alloy and monometallic NPs.