This paper presents the results of study of carbon isotope variation of Paleozoic (including the Cambrian. Ordovician, Devonian, Carboniferous and Permian) marine carbonate stratigraphic sequences. These sections studied were selected at Dabeiwang and Xuzhou, Jiangsu province; Huanghuachang and Yichang, Hubei province; Duan and Debao, Guangxi province; Nashui and Luodian, Guizhou province and Xiangbo and Longling, Guangxi province, respectively. These sections are characterized by successive sequence, completely or mainly consisting of marine carbonate rocks, definition of geologic time and no influence of tectonic, magmatic and metamorphic processes. The sampling distance of these section is 1-3m and 681 samples have been analysed. These samples were treated by commonly used phosphoric acid method. Pure CO2 gas can be collected and analysed for carbon isotope by MAT 251 mass spectrometer. TTB-I (Ordovician limestone collected at Zhoukoudian, Beijing)serves as a working standard. Its δ13C-value is 0.58‰ to PDB. Precision of the individual measurements was better than ±0.1‰ and overall uncertainties were ±0.1‰.681 δ13C values obtained from these sections construct a more accurate and perfect curve indicating the δ13C value variation of Paleozoic marine carbonate sequences except the Silurian strata for which the available section for carbon isotope stratigraphy has not been found so far. The curve shows that the trend of δ13C value variation in Paleozoic is enrichment of 13C in marine carbonate rocks from the Cambrian, Ordovician and Devonian to the Carboniferous and Permian. The trend is confirmed by a few results obtained by other investigators and may be considered as a world-wide phenmenon The trend was caused by the variation of the amount of burial organic carbon which can be related to the sea-level change and perhaps related to the change of the volume of the mid-oceanic ridge system.