In this paper, a series of nitrogen and hydrogen elements (N-H) co-doped diamond single crystals were synthesized in the NiMnCo-C system with melamine (C3N6H6) additive by temperature gradient growth (TGG) method under high-pressure and high-temperature (HPHT). High nitrogen and hydrogen growth environment obviously reduces the growth rate of the diamond. The synthetic diamond crystals are octahedral shape with a green or dark green color. This results indicate that the morphology of the synthetic N-H co-doped gem-diamond can be well-controlled by TGG method. Fourier transform infrared spectroscopy (FTIR) indicate that high-quality diamond single crystals with nitrogen concentrations up to 2100 ppm have been successfully synthesized and the hydrogen related peaks at 2850 cm(-1) (sp(3)-CH3) and 2920 cm(-1) (sp(3)-CH2-) are found in these synthetic diamond. Meanwhile, with the increasing of C-centers the nitrogen atoms aggregate and form into A-centers in these synthetic diamonds. This is the highest quality of N-H co-doped gem-diamond single crystal synthesized in high nitrogen and high hydrogen growth environment reported so far. (C) 2015 Elsevier B.V. All rights reserved.