Rhombohedral boron ( -rh. boron) was doped with V, Cr, Fe, Co, or Zr to investigate the relationship between occupancies of the doping sites and thermoelectric properties. Doping with V or Cr, which preferentially occupy the A 1 sites, produces an increase in electrical conductivity and a negative Seebeck coe7cient, while doping with Fe or Co, which occupy the A 1 and D sites equally, also increases the electrical conductivity though the Seebeck coe7cient remains positive. Doping with Zr, which does not occupy the A 1 sites, slightly increases the electrical conductivity with a resultant larger Seebeck coe7cient. Regarding Co-and Zr-doubly doped p-type -rh. boron, both properties are determined mainly by Co content. The observed temperature dependence of the electrical conductivity of metal-doped -boron clearly indicates variable range hopping conduction such that both properties increase with increasing temperature up to room temperature, in contrast to the behavior of ordinary metals and semiconductors. The power factors of Co-doped p-type -rh. boron and V-or Cr-doped n-type -rh. boron increased with increasing temperature, and doping with about 1 at.% metal atoms produced materials whose power factor at room temperature is three to four orders of magnitude larger than that of pure -rh. boron. Finally, 1 at.% Co doping decreased the thermal conductivity of -rh. boron from 16 to about 4+2 W/m ' K, with the maximum ZT of p-type Co 1.0 B 105 being subsequently estimated as 1.0;10 ؊6 at room temperature.