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Fe3Al2Si3-based thermoelectric materials have low production costs and are environmentally friendly. However, they are usually synthesized via arc-melting, resulting in poor compositional accuracy. Here, Fe36.5Al23.5 + xSi40−x (x = 1.7, 1.8, 1.9, 2.0, and 2.1) compounds with low thermal conductivity (κ) were prepared via levitation melting. The x = 2.1 composition may be the threshold for a p-to-n-type transformation. A minimum κ of ~3.17 W/mK was obtained in Fe36.5Al25.4Si38.1 at 323 K, a decrease of 42.3% relative to the parent Fe3Al2Si3 compound. Moreover, the largest maximum power factor of ~461 μW m−1 K−2 was obtained in Fe36.5Al25.4Si38.1 at 373 K. Using composition self-tuning, a peak figure of merit of ~0.055 was obtained in Fe36.5Al25.4Si38.1 at 423 K. This approach for realizing low κ and a high compositional accuracy should impact the development of advanced room-temperature thermoelectric materials with narrow bandgaps. [ABSTRACT FROM AUTHOR]