Abstract: The carbonate samples studied in this paper wer e collected from the classic sections of Cambrian and Lower Ordovician in th e Yichang area, Hubei province. The results of C- and O-isotope analysis demon strate that δ18O is smaller than or close to －10‰ and does not correlat e with δ13C, suggesting a basically complete preservation of primary rati os of C- and O-isotopes in these carbonate samples. 　　The δ18O profile from the Cambrian to the Lower Ordovician in the Yichang area drifts positively in dolomite horizons and negatively in non-dolom ite horizons. Such a fluctuation in O-isotopic compositions probably reflects p aleoceanic variations in salinity, i.e. large δ18O indicates increas e in paleosalinity. The Z value is calculated in this study to trace quantitatively the changes of paleosalinity. However, the correlation coefficient is only 0.43 between the Z v alue and δ18O but up to 0.99 between the Z value and δ13C.This mea ns that t he Z value mainly reflects changes in C-isotopic compositions and is not suitab le for the reconstruction of paleosalinity in the investigated area. The δ1 3C profile from the Cambrian to the Lower Ordovician in the Yichang area drift s negatively in anoxic sediments (e.g. in black shales of the lower Shujingtuo F ormation), but negative fluctuations are no t limited to anoxic sediments. They also occur in oxic sediments (e. g. in the S hipai, Xiping and Nanjinguan Format ion). This suggests that δ13C, whether in oxic or anoxic sediments, is po ssibly related to the primary productivity of the ancient marine environ ment s. When the black shales of the Shuijingtuo Formation were deposited, abundant p lanktonic organisms were accumulated to the seafloor. The fresh organic material s from these planktons were then consumed by sulfate-reduction bacterial, leadi ng t o the release of CO2 with lower δ13C and the formation of carbonates by the combination of such CO2 with some metal ions . We believe that the intensity of sulfate-reduction largely depended on the su pply of organic materials, i.e. the more sufficient the organic materials, the more intensive the sulfate-reduction, and therefore the lower the value of δ 13C in carbonates. When the oxic sediments were deposited on the sea floor, the remains of planktonic organisms and benthic algae were oxidized causing the release of light C-isotope from these organic materials into the bottom seawat er. Presumbly, the values of δ13C was very low in the carbonate system of the bottom seawater as long as the supply of dissolved oxygen and organic mater ials were sufficient.