Effect of potassium promoter on phase transformation during the isothermal and temperature programmed reduction of the alpha-Fe2O3 catalyst were systematically investigated by in situ XRD. The reduction pathway of Fe2O3 based catalysts follows the same sequence of alpha-Fe2O3 -> Fe2O3 -> alpha-Fe during the isothermal reduction, in contrast to that of alpha-Fe2O3 -> Fe2O3 -> FeO/alpha-Fe- alpha-Fe during the temperature programmed reduction. The potassium promoter notably inhibits the reduction from alpha-Fe2O3 to Fe2O3 due to the enhanced strength of Fe-O bonds, whereas it either accelerates or has no effect on the formation rate of alpha-Fe crystallites during reduction. The potassium promoter increases the CO conversion during FTS, while enhances C-5+, olefins, CO2 selectivity, and decreases CH4 selectivity.