Abstract: In actual production, the actual grain size of materials in delivery state can not be directly inspected by traditional erosion methods. For the reason, the optimum erosion conditions for GCr15 steel in different heat treatment states were obtained through experiments. It is proved that the original austenite grain boundary can be displayed by optimum erosion method, thus the actual grain size of GCr15 steel in annealing and hot rolling states can be determined.