Structural characteristics, chemical bonds and thermoelectric properties of Al-doped boron carbides are studied through calculations of various structural unit models by using a self-consistent-field discrete variation Xα method. The calculations show that Al atom doped in boron carbide is in preference to substituting B or C atoms on the end of boron carbide chain, and then may occupy interstitial sites, but it is difficult for Al to substitute B or C atom in the centers of the chain or in the icosahedra. A representative structural unit containing an Al atom is ［C-B-Al］ε+-［B11C］ε-, while the structural unit without Al is ［C-B-B(C)］ε--［B11C］ε+, and the coexistence of these two different structural units makes the electrical conductivity increased. As the covalent bond of Al-B or Al-C is weaker than that of B-B or B-C, the thermal conductivity decreases when Al is added into boron carbides. With the electrical conductivity increasing and the thermal conductivity decreases, Al doping has significant effect on thermoelectric properties of boron carbides.