N-type β?FeSi 2 with a nominal composition of Fe 0.98 Co 0.02 Si 2 powders has 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 Fe 0.98 Co 0.02 Si 2 consisted of untransformed mixture of α?Fe 2 Si 5 and ε -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 under magnetic field, SEM, and XRD analyses. Isothermal annealing at 830 ? C in vacuum led to the thermoelectric semiconducting β?FeSi 2 phase transformation, but some residual metallic α and ε phases were unavoidable even after prolonged annealing. Thermoelectric properties were remarkably improved by isothermal annealing due to the transformation from metallic α and ε phases to semiconducting phases.