n-type Fe 0 . 9 8 Co 0 . 0 2 Si 2 powders have been produced by mechanical alloying process and consolidated by vacuum hot pressing. As-milled powders were of metastable state and fully transformed to β-FeSi 2 phase by subsequent isothermal annealing. However, as-consolidated iron silicides consisted of untransformed mixture of α-Fe 2 Si 5 and -FeSi phases. Isothermal annealing has been carried out to induce the transformation to a thermoelectric semiconducting β-FeSi 2 phase. The transformation behavior of β-FeSi 2 was investigated by utilizing DTA, a modified TGA, SEM and XRD analyses. Isothermal annealing at 830 o C in vacuum led to the thermoelectric semiconducting β-FeSi 2 phase transformation, but some residual metallic α- and a-phases were unavoidable even after 96 h of annealing. Microstructures of iron silicides were investigated using SEM and TEM. Thermoelectric properties of β-FeSi 2 materials before and after isothermal annealing were evaluated in this study. It was shown that thermoelectric properties were remarkably enhanced by isothermal annealing due to the transformation from metallic α- and -phases to semiconducting β-phases.