Pure phase NbFeSb_1-xSn_x (where x = 0, 0.04, 0.08, 0.12, 0.16) half-Heusler samples were prepared by direct mechanical alloying followed by spark plasma sintering. The results showed that the substitution of Sb by Sn can effectively enhance the peak figure of merit (ZT) to 0.55 at 923 K in NbFeSb_0.88Sn_0.12 through optimized carrier concentration. To further reduce the lattice thermal conductivity (κ_L), we substituted Nb by Hf to enhance the point defect scattering of phonon transport. A maximum reduction of ~80% in κ_L was observed in Nb_0.8Hf_0.2FeSb_0.88Sn_0.12 when compared to undoped NbFeSb at 573 K. We realized a minimum κ_L of ~2.96 W m⁻¹ K⁻¹ at 673 K and a peak ZT of ~0.82 at 973 K for Nb_0.88Hf_0.12FeSb_0.88Sn_0.12.