The microstructure and magnetic properties of multilayer [FePt(x)/Os] n films on glass and Si substrates by dc-magnetron sputtering technique have been studied as a function of the annealing temperatures between 300 and 800 C. Here, x varied from 10, 20, 25, 50, to 100 nm with its associated n value of 10, 5, 4, 2, and 1, respectively. On glass, no diffusion evidence was found in all the samples. However, on Si, the insertion of a 10 nm Os layer into the FePt and Si interface results in better thermal stability. The Os underlayer can effectively prevent the diffusion of the intermixing between FePt layer and the Si(100) substrate for temperatures up to 800 C. The grain size of the multilayer films can be well controlled by both the annealing temperature and the thickness of the FePt layers between the Os layers. The Os layers can effectively prevent the diffusion of the intermixing among the FePt layers and the Si(100) substrate. This means that the diffusion effect can be efficiently prevent in the multilayer [FePt(x)/Os] n films by the Os layers.