The influence of Ag on the magnetic properties of COPt3 nanoparticles has shown that the addition of Ag (5.0 mol %) can not change the size and crystal structure (i.e., fee structure) of COPt3 nanoparticles. However, SQUID studies have revealed that the addition of Ag can notably improve the magnetic properties of COPt3, which could result from the interaction between Ag and COPt3 nanoparticles.