(Ni66Fe22Co12)(x)C1-x nanocomposite films with x = 10-75 at.% were prepared by d.c. magnetron co-sputtering. Subsequent thermal annealing was performed in a vacuum (<2X10(-3) Pa) furnace for 1 h at various temperatures. The microstructure evolution of the as-deposited and annealed films was characterized by non-Rutherford backscattering spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, Raman spectroscopy, and atomic force microscopy. Results showed that the phase transition with the increase of annealing temperature was closely dependent on the composition. Films with less than 20 at.% NiFeCo showed an amorphous structure in the as-deposited and up to 400C annealed samples. After annealing at 500 degreesC, a small amount of face-centered-cubic (fcc) crystalline NiFeCo precipitated while carbon remained amorphous. For the films with NiFeCo concentration in the range of 30-55 at.%, the as-deposited films were found to consist of very small NiFeCo nanocrystals encapsulated in amorphous carbon. After annealing, the crystal grain size of the alloys increased with the increase of annealing temperature while carbon was graphitized. For the films with more than 62 at.% NiFeCo, the as-deposited films went through a meta-stable stage at which a rhombohedral Ni3C phase and fee NiFeCo co-existed upon annealing to a temperature between approximately 300 and 400 degreesC (dependent on composition). Upon further annealing to a sufficiently high temperature between approximately 350 and 500 degreesC, the carbide phase decomposed and only the fee NiFeCo nanocrystals encapsulated in graphite existed in the films.