The style, decoration and inscription of the bronzes have been studied for determining the foundry area of the bronze vessels, which is subject to the subjective experience and knowledge of archaeologists. Some scholars have analyzed lead isotope and trace elements of bronze vessels and attempted to ascertain the foundry area of the bronzes using natural science means. However, the signature of lead isotope and trace elements of the bronzes provide information on ore sources whose metal minerals were used to cast the bronzes, not the information on the foundry area. The composition of casting clay core might provide clue to the foundry area of the bronze vessels. This paper probed into the casting place of the bronze vessels on the basis of an analysis of clay core residues of the bronze vessels. Samples of the fragments of the casting clay cores were removed from the bronze vessels unearthed at Jiuliandun tombs of Chu State, in Zaoyang City, Hubei Province, dated back to the Warring States Period, and were analyzed by such means as polarized light microscope, XRD, XRF, NAA and ICP-AES. The results reveal that the casting clay cores of extraneous bronze vessels from Jiuliandun tombs are different from those of local bronze vessels excavated in No. 1 and No. 2 Jiuliandun tombs in such aspects as phase composition, microstructure, major elements, trace elements, REE and phytolith assemblages. The content of CaO in casting clay cores of extraneous bronze vessels is very high and the content of Na₂O is low, which are representative characteristics of the loess in northern China. Combined with the analysis of phytolith assemblages, it is inferred that extraneous bronze vessels were cast in the loess area of northern China. This study also indicates that it is feasible to restrict the possible foundry area of the bronze vessels according to the analysis of casting clay cores residues on the bronzes.