Fine particles of Fe0.92Mn0.08Si2.00, 200 nm in size, were produced using a high-frequency plasma. The powders were sintered at temperatures between 1323 and 1423 K in vacuum and then heat treated at 1073 K to transform them into thermoelectric semiconducting iron disilicide. The relative density of the samples was as low as 70-77%, and the microstructure was composed of several tens of micrometre grains and continuous pores. The sample sintered at 1423 K, in particular, showed a figure of merit, Z, of 2.0 x 10(-4) K-1 at 870 K which was 20% larger than those previously reported. The increase in Z was found to be attributable to the increase in Seebeck coefficient and the decrease in thermal conductivity.