First-principles calculation was performed to investigate the effect ofmetal doping on the structural characteristics of amorphous carbon system, and the 3d transition metals (TM) were particularly selected as representative case. Results showed that the total energy in TM-C systems caused by distorting the bond angles was reduced distinctly for comparison with that in C-C system. Further electronic structure revealed that as the 3d electrons of doped TM increased, the bond characteristic of highest occupied molecular orbital changed from bonding (Sc, Ti) to nonbonding (V, Cr, Mn, Fe) and finally to antibonding (Co, Ni, Cu) between the TM and C atoms. Meanwhile, the TM-C bond presented amixture of the covalent and ionic characters. The decrease of strength and directionality of TM-C bonds resulted in the total energy change upon bond angle distortion, which demonstrated that the bond characteristics played an important role in reducing residual stress of TM-doped amorphous carbon systems. (C) 2016 Elsevier B.V. All rights reserved.