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Magnetic properties and nanostructure of FePt : C cluster-deposited films with C volume fraction of 7%, 14%, 33%, and 45% have been studied. As-deposited FePt : C films were prepared by a multilayer method in which FePt layers were deposited from a cluster source employing a gas-aggregation technique and C layers from a normal sputtering gun. In the as-deposited films, FePt clusters with fcc structure are embedded in the C matrix. The high anisotropy FePt L10 cluster structure was realized in the films via post-deposition annealing and the nanostructure of the films was observed by high-resolution transmission electron microscope (TEM). The results for a film with 45 vol. % C showed that FePt clusters are well separated by C matrix and the cluster diameter is about 4.5 nm. The coercivity increases with increase of annealing temperature; coercivities larger than 9 kOe were achieved in the films after annealing at a temperature of 700 ºC and above. Magnetization reversal of the films was studied by moment-decay measurements and the data were fitted with the Sharrock formula. For the film with 45 vol. % C annealed at 625 ºC, the thermal stability factor KuV*/kB, T activation volume V*, and anisotropy constant Ku are 231, 0.83 ×10-18cm3 and 1.2 ×107 erg/cm3, respectively.