The effect of addition of B or C on thermoelectric properties of YbAl 3 was investigated. YbAl 3 is well known as one of heavy fermion intermetallic compounds, which generally possess relatively large Seebeck coefficient and extremely low electrical resistivity. Yb 1.05 Al 3 M x (M=B or C, x=0–0.10) showed almost single phase with AuCu 3 -type structure. The Seebeck coefficient and electrical resistivity of Yb 1.05 Al 3 were hardly affected by the addition of B or C, which showed that there was no difference in the carrier density among these compounds. The addition of C, or especially B, was effective for the reduction of the thermal conductivity, which was mainly attributed to the reduction of the lattice contribution. As a result, Yb 1.05 Al 3 B 0.10 showed a maximum ZT of about 0.33 at 323K, which was a little larger than ZT of Yb 1.05 Al 3 .