A series of Co-doped amorphous carbon granular thin films with various Co contents (x, at%) were deposited on n-type Si (100) substrates by a magnetron co-sputtering technique at room temperature. The microstructure, magnetotransport and magnetic properties of the Co-C films have been characterized. By optimizing Co content, giant negative magnetoresistances (MRs) were observed at low temperature, with a maximum MR ratio of 27.6% at 2 K under an applied magnetic field of 90×79.6 kA·m-1 for the film with Co content of 6.4 at%. As Co content increases from 6.4 at% to 16.4 at%, the MR ratio decreases gradually from 27.6% to 2.2%. A linear lnr-T–1/2 relationship indicates that the spin transport in Co6.4C93.6 thin film agrees well with tunneling conductance.