We investigated the effect of loading potassium over iron-based catalysts for water-gas shift (WGS) reaction at 573 K. The iron-based water-gas shift catalyst has been known as a redox-mechanism catalyst. Therefore, to promote the redox reaction over iron oxide, we impregnated a small amount of Pd on various iron oxides. Results revealed that coexisting potassium and palladium accelerated the WGS reaction over iron-oxide catalyst. We examined the optimum amount of potassium over Pd/iron catalyst, and we found that the optimum molar ratio was about 2 (K/Pd molar ratio). From the viewpoint of reducibility of the catalyst, the addition of small amount of Pd onto iron oxide promotes reduction from Fe2O3 to Fe3O4. However, impregnation of potassium on iron oxide makes the catalyst structure robust. The promotion effect of Pd and potassium was not observed on SiO2 support. We therefore concluded that the synergetic effect among Pd, K, and iron oxide, was important for the WGS reaction.