We synthesized divalent hexaborides (MB₆: M=Ca, Sr, and Ba) and examined their thermoelectric (TE) and transport properties to discuss methods of improvement in their TE performance. Hall measurements showed that the carrier concentration of the MB₆ increases with the M’s atomic number, while the carrier mobility is almost constant among the divalent hexabories we examined. In addition, substitution of part of the alkaline earth metal with one of the others changes carrier concentration but doesn’t change the mobility. As the carrier concentration increases, Seebeck coefficient decreases and electrical conductivity increases. These results imply that TE properties of the divalent hexaborides depend only on the carrier concentration. And we found that the maximum power factor can be obtained at carrier concentration of around 2x10^-20 cm^-3. For the further improvement of their TE performance, the remaining key factor of thermal conductivity should be reduced. The substitution of metal atom with maintaining the best carrier concentration, namely the best power factor, will be effective to reduce the thermal conductivity, resulting in the improvement of TE performance. Detailed results and discussions will be presented at the conference.