In the Fe-Si system, a mixture of a($Fe_{2}Si_5$) - and ${epsilon}$(FeSi)-composition powders was sintered and heat-treated subsequently at various temperatures and time to get thermoelectric ${eta}$-phase($FeSi_2$) compacts. The different transformational sequences depending on the heat treating temperature were found through the investigation into phase transformation and microstructural development. That is, a rapid eutectoid decomposition of ${alpha}{ o}{eta}+Si$ occurred together with a accompanying slow reaction between the dispersed Si formed by above decomposition and the preexisted ${epsilon}$ phase at temperatures below $830^{circ}C$. The unreacted Si and the micropores formed due to the density change upon the transformation coarsened as heat treating time elapsed. At temperatures above $880^{circ}C$, however, transformation was proceeded by a peritectoid reaction of ${alpha}+{epsilon}{ o}{eta}$. It took at least 200min. to achieve 90% volume fracion of transformed ${eta}$ phase, and the growth of micro-pores was also observed in this transformational sequence with prolonged heat treating time.