High temperature confocal melting was conducted to study the volume fraction variation of as-cast δ-ferrite at cooling rate from 5 °C/min to 6000 °C/min of 316H austenitic stainless steel (316H). The results demonstrate that the solidification mode of 316H was ferrite-austenite type (FA mode) regardless of the cooling rate. The peritectic reaction occurs between ferrite and liquid phase during solidification. The higher the cooling rate is, the higher the δ-ferrite volume fraction is. The δ-ferrite fraction variation can be explained by the combination of FA solidification mode during solidification and ferrite to austenite solid-state transformation during cooling after solidification.