Effects of charge state and interatomic distance on the magnetic properties of C-doped MgO system are explored by a revised PBE (PBEsol) functional plus U method. Both PBEsol and PBEsol +U results demonstrate that single neutral C substituted at the O site can induce a stable magnetic moment of 2 mu(B) into the doped system. We mainly find that the size of the magnetic moment is determined by the charge states of the C dopant and strongly depended on the distance between two C dopants. These results are expected to stimulate future experimental work on the dilute magnetic semiconductors. (C) 2014 Elsevier B.V. All rights reserved.