Due to the lack of index fossils and extensive alteration, the geological age of the Jeongseon Formation, Jeongseon Unit of the Joseon Supergroup (lower Paleozoic) is yet clearly defined by any chronostratigraphic mean. In this study, we present a high-resolution carbon isotopic curve for the upper part of the Jeongseon Formation. The carbon isotopic values of the Jeongseon Formation fluctuate between −3%. and +1%., which are within the range of those of Middle Ordovician carbonate. Also, the δ¹³C curve of the upper part of the Jeongseon Formation is well correlated with that of the uppermost Middle-lowermost Upper Ordovician Copenhagen Formation in the Great Basin, U.S.A. (Saltzman and Young, 2005). Considering general biostratigraphy and carbon isotope stratigraphy together, the age of the upper part of the Jeongseon Formation can be assigned to late Middle to early Late Ordovician.