To date, solar cells with efficiency of 12.6% has been demonstrated via a hydrazine-based solution approach. Despite this progress, performance of Cu₂ZnSn(S,Se)₄ solar cells remains far lower than the Shockely- Quiser theoretical limit. We performed density functional theory calculations with hybrid functional approach to investigate the Mg-related defects in the kesterite structure of the Cu₂ZnSnS₄ (CZTS) solar cell material. The substitution energies of Mg atom in CZTS were calculated in consideration of the atomic chemical potentials of the constituent elements of Cu, Zn, Sn, and the doping atom of Mg. From our calculation results, Mg doping in CZTS under certain Sn-rich growth condition is expected to convert the conduction from p-type to n-type. The present study provides a theoretical basis for exploring practical applications of Mg doping in CZTS solar cells.