Performing density functional theory calculations, we have studied the adsorption of nanoparticle Co-n (n = 2, 4, 6) on graphene sheets and Co-4 stands out for the most favorable stability. Furthermore, the catalytic activity of Co-4-supported on a variety of graphene sheets was explored toward ammonia dehydrogenation. Present results demonstrate that the existence of C-vacancy in graphene considerably enhances the interaction of NHx (x = 3-0) with Co-4 cluster, and effectively facilitates H abstraction from ammonia. Specifically, Co-4 anchored on mono-vacancy graphene sheet outperforms the others for its superb catalytic activity in NH3 decomposition, giving rise to the intermediates NH and/or N bound with the nanocomposite. In contrast, doping of N or B atom in graphene hinders the N-H bond cleaving of NH3. The higher catalytic activity of Co-4 supported over the vacancy graphene is correlated well with the lower d-band center value relative to the Fermi level, consistent with the literature emphasizing on the importance of support to metal catalysts.