We have been investigating the high-temperature thermoelectric properties of some novel rare earth borides with a structure containing B 12 icosahedra. Doping effects on the TE properties in such systems were investigated for the first time. A series of Nb-doped YB 66 and C-doped YB 66 single crystals were grown by the floating zone method. The Nb-doped compounds have approximate chemical formulas ranging from YNb 0.30 B 66 to YNb 0.33 B 66 while the C-doped compound has a formula of YB 66 C 0.6 . The effect of Nb-doping on the thermoelectric properties was not monotonic and appears to be complex. As a result of Nb-doping, the room temperature resistivity and the characteristic temperature T 0 were considerably reduced. At room temperature the power factor of the Nb-doped YB 66 sample with 89% site occupancy was three times greater than that of non-doped YB 66 . However, in the important high-temperature region, the nondoped sample actually exhibited the highest power factor for T4550 K. Furthermore, owing to a structural feature of YB 66 , thermal conductivity actually increases with doping of transition metals. Taking into account all the thermoelectric properties, transition metal doping of YB 66 is therefore not suitable for our purposes. On the other hand, doping of carbon, which is assumed not to go into the same sites as the transition metals, yielded a lowering of the thermal conductivity. Furthermore, contrary to Nb-doping, carbon doping did not result in a reversal of the relative magnitude of resistivity at extremely high temperatures and therefore, an increase in the figure of merit of factor 2 was realized at 1000 K.