Herein, Sn was successfully doped into the Sb site of n-type NbCoSb half-Heusler compounds to tune the carrier concentration, and a maximum ZT value of ∼0.56 was obtained at 973 K for NbCoSb1-xSnx with x = 0.2, an increase of ∼40% as compared to that of NbCoSb. This enhancement is mainly attributed to the reduced carrier concentration by Sn doping, leading to a doubled Seebeck coefficient at 300 K. More importantly, the total thermal conductivity was reduced with Sn doping, and the reduction was mainly due to the lowered electron thermal conductivity. The decreased electron thermal conductivity resulted from the reduced carrier concentration and the consequent enhanced carrier degeneracy, contributing to a reduced Lorenz constant. A quantitative description of the electron transport characteristics was performed under a single parabolic band model supposing that the acoustic phonon scattering was dominant in the carrier transport. A high density of the state effective mass, m* ≈ 10 me, and relatively high deformation potential Edef = 21 eV were found for the solid solutions.