Recently, we found a new n-type thermoelectric half-Heusler NbCoSb with a valence electron count of 19, different from the usual 18. In this paper, we focus on the effect of partial substitution of Nb by isoelectronic elements V and Ta on the reduction of the thermal conductivity of Nb_(1−x)/2V_(1−x)/2Ta_xCoSb. We found that the isoelectronic elements V and Ta for partial substitution of Nb can dramatically decrease the thermal conductivity from 7.0 W m⁻¹ K⁻¹ to 3.3 W m⁻¹ K⁻¹ at room temperature for Nb_0.44V_0.44Ta_0.12CoSb, but unfortunately a large power factor decrease also occurred. Consequently, a peak ZT of ∼0.5 is achieved at 700 °C for Nb_0.44V_0.44Ta_0.12CoSb, which is about 25% higher than the ∼0.4 reported earlier for NbCoSb.