In this study,we investigate the effect of nitrogen and hydrogen impurities on colors,morphologies,impurity structures and synthesis conditions of diamond crystals in Fe-C systems with C3N6H6 additives at pressures in the range 5.0-6.5 GPa and temperatures of 1400-1700 ℃ in detail.Our results reveal that the octahedron diamond nucleation in a Fe-C system is evidently inhibited by co-doped N-H elements,thereby resulting in the increase of minimum pressure and temperature of diamond synthesis by spontaneous nucleation.The octahedron diamond crystals synthesized from a pure Fe-C system are colorless,while they become green in the system with C3N6H6 additive.The surface defects of diamond will deteriorate when the nitrogen and hydrogen atoms simultaneously incorporate in the diamond growth environment in the Fe-C system.We believe that this study will provide some important information and be beneficial for the deep understanding of the crystallization of diamonds from different component systems.