Ammonia (NH3) has been considered as a promising alternative for hydrogen storage. In this work, the decomposition mechanism of ammonia was studied on various Pd-doped Cu(1 0 0) surfaces by DFT calculations. It was found that the coverage of Pd atoms on Cu(1 0 0) surface made an evident influence on the interaction between the NHx and substrate. The doped Pd atoms significantly promoted NH3 decomposition kinetically and thermodynamically compared to Cu(1 0 0), confirming the synergistic effect between Pd and Cu. The catalytic activity and selectivity of various Pd-Cu(1 0 0) surfaces for NH3 decomposition were dependent upon the coverage of Pd atoms to some extent.