The present work mainly evaluates the effect of potassium (K) on as-cast microstructure of a hypereutectic high chromium cast iron by means of a field emission scanning electron microscope (FESEM) and an X-ray diffractometer using CuKα radiation with a 2θ range of 30-130°. Results showed that, with the addition of K-containing modifier, the large lath-like and/or rod-like primary M7C3 carbides can be modified to the hexagonal prisms, and the eutectic carbides can also be refined. In addition, the carbides are distributed much more homogeneously in the matrix. The modification effect of K is due to its aggregation at the liquid-solid interface and the adsorption on the relatively fast growing planes during the solidification, which influence the growth rates of different crystal planes and lead to the modification of the carbides.