In this paper, we report on the structural properties and superconductivity of Mg(B1-xCx)2 compounds. PowderX-ray diffraction results indicate that the samples crystallize in a hexagonal AlB2-type structure. Due to the chemicalactivity of Mg powders, a small amount of MgO impurity phase is detected by X-ray diffraction. The lattice parametersdecrease slightly with the increasing carbon content. Magnetization measurements indicate that the non-stoichiometryof MgB2 has no influence on the superconducting transition temperature and the transition temperature width. Theaddition of carbon results in a decrease of Tc and an increase of the superconducting transition width, implying the lossof superconductivity.