The effects of alloy grain size of high-Cr ferritic stainless steel were investigated using a sheet (t = 2 mm) and plate (t = 22 mm) of Cr ferritic stainless steel. The average grain size of each sample was measured as ∼ 25 µm and ∼ 500 µm, respectively. The large-grained plate sample exhibited a 41% oxidation rate reduction, and a slight decrease in Cr volatilization was observed in the small-grained sheet sample. A numerical examination revealed that inhomogeneous GB-related oxidation is a major cause of the different oxidation kinetics affected by metal grain size.