Cobalt (Co) is a potential candidate in replacing copper for interconnects and has been applied in the trenches in semiconductor industry over twenty years. A non-oxidizing reactant is required in plasma-enhanced atomic layer deposition (PE-ALD) of thin films of metals to avoid O-contamination. PE-ALD of Co has been demonstrated experimentally with plasma sources of NH3 or a mixture of N2 and H2, but the growth mechanism and key reactions are not clear. In this paper, we have investigated the reactions of plasma generated radicals H, N, NH and NH2 at metal precursor (CoCp2) terminated Co (001) and (100) surfaces using static DFT calculations at 0 K and molecular dynamics simulations at 600 K. N radicals play an important role in eliminating surface-bound Cp ligand (if any) via pyridine (C5H5N) formation and desorption, whereas H radicals have endothermic reactions for eliminating Cp ligand via CpH formation and desorption. The surface NHx species are eliminated by H radicals via NH3 formation and desorption. The simulations of these key reactions show that on Co(001) surface, the remaining Cp ligand and surface NHx species after the metal precursor pulse will be completely removed with N and H radicals, resulting in Co atoms deposited on Co(001) surface at a coverage of 3.03Co/nm2. Whereas, on Co(100) surface, the surface NH2 species cannot be completely removed via NH3 formation and desorption due to overall endothermic reactions. Instead, H radicals react with trench N species, contributed to H transfer at metal precursor pulse, to form NH. These trench N species cannot be eliminated completely on Co(100) surface, which will be the source of N impurities for the deposited Co thin films. At the post-plasma stage, the metal surface will be covered with NHx-terminations with plasma generated NH radicals, which is then ready for the next deposition cycle. Our results explain why ammonia or H2/N2 plasma, which produce NHx species are required to deposit Co thin films using Co metallocene precursors.