An effective way is developed to fabricate AgPt alloy nanoislands oil gold nanorods based on the galvanic replacement between Ag and PtCl42-in the presence of cetyltrimethylammonium bromide (CTAB). The optical and catalytic properties benefit from the porous structure composed of AgPt nanoislands. A large red shift (265 nm) after etching is observed for longitudinal surface plasmon resonance (SPR) in comparison with [email protected]@Ag. Alloy compositions ill bulk miscibility gap can be obtained and finely tuned from Ag0.56Pt0.44 to Ag0.38Pt0.62. A unique composition dependence is found for both electrocatalytic oxidation of methanol and catalytic oxidation of o-phenylenediamine (OPD) by hydrogen peroxide. In both systems, the highest catalytic activity is achieved at the alloy composition of Pt0.62Ag0.38. Proper alloying with Ag not only improves the CO poisoning of Pt catalyst but also enhances the catalytic activity greatly.