In order to develop a new application of cast iron, high niobium cast iron has been developed as a grain refiner for stainless steel. High niobium cast iron was prepared by adding pure niobium to a commercial cast iron. Coarse primary niobium carbide crystals were observed in the microstructure of the cast iron. The effect of the high niobium cast iron as an inoculant on the grain size of austenitic and ferritic stainless steels was examined in various experimental conditions. When the amount of the cast iron inoculant more than 3 mass% was added into the steel melt, fine equiaxed grains were observed and grain size was significantly reduced to 210 μm. The results indicate that the high niobium cast iron is effective as a grain refiner for the austenitic and ferritic stainless steels. From the dissolution rate measurement, the grain refining mechanism was proposed.