The addition of Co to Nd4Fe77.5−x Co x Hf0.5Ga0.5B18.5 (0≤x≤5) was found to enhance the magnetic properties of Fe3B/Nd2Fe14B nanocomposite magnets. The enhancement resulted from the fact that Co tends to retard the formation of Fe3B from the amorphous matrix but to accelerate that of Nd2Fe14B. The decreased interval between the crystallization temperature of Fe3B and Nd2Fe14B led to a uniform grain size distribution of both phases during the annealing treatment. The additive Co was confirmed to partition mainly to Nd2Fe14B crystals rather than to Fe3B which was traced by XRD and Mössbauer spectroscopy as well. About 72 vol.% of Fe3B, 27 vol.% of Nd2Fe14B, and a small amount of Fe around 1 vol.%, respectively, were found to form. However, the volume fraction of each phase did not vary by the addition of Co up to 5 at.%.