Herein we report a significantly reduced lattice thermal conductivity of Sb-doped Hf_0.35Zr_0.35Ti_0.3NiSn half-Heusler alloys with sub-micron grains (grain size of ~300 nm). Polycrystalline bulks of Hf_0.35Zr_0.35Ti_0.3NiSn_1−xSb_x (x = 0.01, 0.02, 0.03) with a complete single half-Heusler phase are prepared using temperature-regulated melt spinning and subsequent spark plasma sintering without a long annealing process. In these submicron-grained bulks, a very low lattice thermal conductivity value of ~2.4 W m⁻¹ K⁻¹ is obtained at 300 K due to the intensified phonon scatterings by highly dense grain boundaries and point-defects (Zr and Ti substituted at Hf-sites). A maximum thermoelectric figure of merit, zT, of 0.5 at 800 K is obtained in Hf_0.35Zr_0.35Ti_0.3NiSn_0.99Sb_0.01.