Transformation of the “core” atomic structure and electronic states of the tilt Σ5 θ = 37° [001]/(130) grain boundary in poly-Si due to incorporation of carbon atoms into the oxygencontaining complexes is studied using MM and MO LCAO methods. Different numbers n = 1 ÷ 4 of C-atoms were introduced into the 5-fold interstitial positions in the initial O-containing complexes built-up from SiO3 and SiO4 configurations at the GB “core”. Incorporation of C-atoms into SiO3 and SiO4 complexes leads to the formation of Si-O-C-Si chains and shifting of the donorlike levels generated by SiO3 and SiO4 configurations to the bottom of the conduction band with an increase in the number of the incorporated C-atoms.