The transition from Schottky to ohmic contact in nickel silicide/SiC system during annealing from 600 to 950 degreesC was investigated by measuring the electrical proprieties of the contact and by analyzing the microstructure of silicide/SiC interface. The graphite clusters formed by carbon atoms during silicidation are uniformly distributed into the silicide layer after annealing at 600 degreesC and they agglomerate into a thin layer far from the silicide/SiC interface after annealing at 950 degreesC. At this temperature an increase of about 0.25 eV of the Schottky barrier height was measured, while Deep Level Transient Spectroscopy evidences an evolution of interfacial defects. The peak related to C-vacancy sited at 0.5 eV from the conduction band has not been observed. This result invalidates the reported literature hypothesis that the formation of ohmic contact after annealing at 950 degreesC was related to the formation of C vacancies.