The mechanism for formation of a weld line in slip casting of large potteries was clarified to prevent the occurrence of cracks in the dried green body. It was found from the observation of flow of the slurry during the slip casting that the weld line was formed by contact between surfaces of slurry in which the moisture content was decreased by the contact with the inner surface of the gypsum mold. The formation of weld line was prevented by vibrating the gypsum mold during slip casting.