| Summary: | In this study, FeSi<sub>2</sub> bulk specimens were prepared by mechanical alloying, spark plasma sintering, and subsequent annealing. The annealed FeSi<sub>2</sub> bulk specimens consisted of the β-FeSi<sub>2</sub> phase and exhibited high Seebeck coefficient values. The maximum Seebeck coefficient of 356 μVK<sup>−1</sup> was achieved in the FeSi<sub>2</sub> bulk specimen annealed at 1173 K for 6 h. However, the power factor of the FeSi<sub>2</sub> bulk specimen was quite small due to its high electrical resistivity, and a drastic improvement is required. Therefore, Mn- and Co-substituted specimens, Fe<sub>1−x</sub>Mn<sub>x</sub>Si<sub>2</sub> (x = 0.2–0.8) and Fe<sub>1−x</sub>Co<sub>x</sub>Si<sub>2</sub> (x = 0.2–0.8), were produced, and their thermoelectric properties were evaluated. The Mn- and Co-substituted specimens exhibited lower electrical resistivity and a higher power factor than the FeSi<sub>2</sub> bulk specimen. The Fe<sub>1−x</sub>Mn<sub>x</sub>Si<sub>2</sub> (x = 0.2–0.8) bulk specimens were p-type thermoelectric materials, and a Seebeck coefficient of 262 μVK<sup>−1</sup> and a power factor of 339 μWm<sup>−1</sup>K<sup>−2</sup> were achieved in the Fe<sub>0.94</sub>Mn<sub>0.06</sub>Si<sub>2</sub> bulk specimen. On the other hand, the Fe<sub>1−x</sub>Co<sub>x</sub>Si<sub>2</sub> (x = 0.2–0.8) bulk specimens were n-type thermoelectric materials, and a Seebeck coefficient of −180 μVK<sup>−1</sup> and a power factor of 667 μWm<sup>−1</sup>K<sup>−2</sup> were achieved in the Fe<sub>0.96</sub>Co<sub>0.04</sub>Si<sub>2</sub> bulk specimen.
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