Silver (Ag) is a typical catalyst for CO2 electroreduction. To improve the CO faradaic efficiency, ZIF-8 supported thiol-capped Ag nanoparticles (Ag NPs) of about 3.9 nm were facilely synthesized by reducing Ag + in ZIF-8 suspension in the presence of thiol-containing mercaptopropionic acid (MPA) molecules. MPA can induce the monodisperse Ag NPs formation, cap on the surface of Ag NPs and immobilize on ZIF-8 through coordinating with the unsaturated Zn 2+ cations. The Ag/ZIF-8 can efficiently reduce CO2 to CO with a maximum CO faradaic efficiency of 92.3% at the potential of −0.88 V (vs. RHE), higher than that of ZIF-8 and Ag/C. The performance improvement in CO2 electroreduction over Ag/ZIF-8 is probably attributed to the small-size of Ag NPs and the synergistic effect between Ag NPs and ZIF-8.