Thermoelectric performance of InSb is restricted by its low Seebeck coefficient and high thermal conductivity. Here, CuCl is employed to optimize simultaneously the electrical and thermal transport properties of InSb. The substitution of Cl for Sb results in enhanced electron effective mass, leading to high Seebeck coefficient of–159.9 μV/K and high power factor of 31.5 μW⋅ cm− 1⋅ K− 2 at 733 K for InSb+ 5 wt% CuCl sample. In addition, CuCl doping creates hierarchical architectures composed of Cu 9 In 4, Sb, Cu 2 Sb in InSb, leading to a strengthened phonon scattering in a wide wavelength (ie, nano to meso scale), thus a low lattice thermal conductivity of 2.97 W⋅ m− 1⋅ K− 1 at 733 K in InSb+ 5 wt% CuCl. As a result, a maximum ZT of 0.77 at 733 K has been achieved for the InSb+ 5 wt% CuCl sample, increasing by∼ 250% compared to pristine InSb.