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BiSbSe 3 is a promising medium-temperature thermoelectric material on account of its intrinsically low thermal conductivity. To further enhance the thermoelectric performance in n -type BiSbSe 3 , we conduct Br doping at Se sites to optimize its carrier concentration and electrical conductivity, finally a maximum ZT of ∼0.8 in nanostructured BiSb(Se 0.94 Br 0.06) 3 is obtained at 700 K through mechanical alloying. Based on the optimal BiSb(Se 0.94 Br 0.06) 3 composition, we perform percolation effect with mixed grain sizes in nanoscale and microscale to enhance carrier mobility. It is found that carrier mobility is favorably improved by the addition of microscale grains (∼30 μm) in nanostructured BiSb(Se 0.94 Br 0.06) 3 matrix due to the reduced grain boundaries to carrier scattering, which benefits high electrical conductivity and power factor. Simultaneously, the BiSb(Se 0.94 Br 0.06) 3 matrix with mixed grain sizes still maintains very low thermal conductivity due to its intrinsically strong lattice anharmonicity. Owing to the optimized electrical properties and maintained low thermal conductivity, we obtain a maximum ZT value of ∼1.0 at 700 K in BiSb(Se 0.94 Br 0.06) 3 –4C matrix with 40% microscale grains. This work provides a feasible method to optimize thermoelectric performance by designing microstructure with percolation effect. • Carrier concentration was improved to ∼1.91 × 1020 cm−3 in n -type BiSbSe 3 by Br doping. • PF max of n -type BiSbSe 3 was significantly enhanced to ∼6.0 μW cm−1 K−2 at 700 K through improving carrier mobility via utilizing percolation. • ZT max of n -type BiSbSe 3 was enhanced from ∼0.1 to ∼1.0 at 700K. [ABSTRACT FROM AUTHOR]