The aim of this paper was to discuss the differences between masking layer in porcelains produced during Tang Dynasty, the Five Dynasties era and the white middle layer (between the body and glass) during the Northern Song Dynasty in the Yaozhou kiln, one of the most famous kilns in northern China. EDXRF was used because of its microanalytical capability. The results indicated that the chemical components of middle layer of Northern Song porcelains were different from those of the Tang and Five Dynasties periods. The content of Fe2O3 in the white middle layer of the Song pieces was lower than those of body and glass. Furthermore the contents of CaO and K2O in the white middle layer were both higher than those of the masking clay but close to those of white masking clay. As for the middle layer, the content of Fe2O3 was little lower than in the clay body, but other components were close to those of body. The content of Fe2O3 in the middle layer was lower than that of the masking clay.