The purpose of the present experiments is to consider the relations between coloration and crystalline phases of lime-glaze, dolomite-glaze and lime-barium-glaze containing 15 mass% ferric oxide and 15 mass% phosphate. The result of the experiments showed that red color under hard oxidizing fire only appeared in the glaze with MgO, Fe₂O₃ and P₂O₅. As a result of X-ray diffractometry, the colour of red was exposed by magnesioferrite-crystal. However, magnesioferrite-crystal can exist as whitlockite-type crystals. The hard oxidizing fire iron red glaze is one of the crystalline glazes which is colorized by magnesioferrite during cooling process from firing. In addition, magnesioferrite crystal formation is closely related to the presence of whitlockite-type crystals.