Transition-metal nitrides have long attracted considerable attention among researchers and ubiquitous applications in various fields due to their renowned mechanical properties. However almost all the discussions of the strengthening mechanism were on conventional meso scale. For further understanding on the atomic scale strengthening mechanism of transition-metal nitrides, three groups of MN_x (M=Ti, Zr, Hf) films with different nitrogen contents were synthesized on the Si substrates by magnetic filtering arc ion plating. The morphologies and thickness of the as-deposited films were characterized by FESEM, the microstructures and the residual stresses were characterized by XRD, the XPS and Nano Indenter were used to measure the chemical states and hardness (also the elastic modulus) of as-deposited films, respectively. The results show that all three groups MN_x films perform the B1-NaCl single-phase structure within the large composition ranges. The preferred orientation, thickness, grain size and residual stress of the MN_x films with different nitrogen contents were not changed so much. While the nanohardness and elastic modulus of MN_x both first increased and then decreased with the rise of nitrogen content, and the peak values all existed when x near to 0.82. The strengthening mechanism was discussed and the decisive factor of composition dependent hardness enhancement was found from the atomic-scale chemical bonding states and electronic structure in this work, rather than the conventional meso-scale factors, such as preferred orientation, grain size and residual stress.