Objectives: Ooids are the important materials for reconstruction of the palaeoseawater chemistry and the palaeoenvironment, but little attention has been paid to the ooids in the Dengying Formation. As we all know, the Dengying stage is a vital period for the evolution of life on the earth, however the seawater chemical compositions and paleoenvironment during this period remain controversial. To pave these researches, this paper studied petrological characteristics of the ooids from the upper Dengying member II in detailed, particular attention was focued on the sedimentary environment, time of the dolomitization and primary mineralogy of the ooids, to solid the foundation for future researches about palaeoseawater chemistry and palaeoenvironment. Methods: The ooids development in field and petrological characteristics and ooid sizes were studies through the outcrop observing and microscope analysis. Results: The ooids developped in the upper Dengying member II can be grouped into two oolitic segments which were separated by a 2.4 m thick mud—silt crystal dolomite horizon. The oolitic segment overlies on the mud—silt crystal dolomite horizon is about 1.4 m thick while the lower one is about 5.5 m thick. Five types of ooids were identified (i.e. mud—microcrystalline ooids, radial—concentric ooids, concentric ooids, radial ooids and oher type ooids). Among them, the mud—microcrystalline ooids are most abundant, the concentric ooids and radial—concentric ooids come second, whereas other types were less observed. Conclusion: Based on field observation and petrological analysis, we conclude that ooids in study area were deposited in oolitic shoal setting. They were deposited in different hydrodynamic conditions. The radial—concentric ooids and concentric ooids indicate that they were formed in strong hydrodynamic conditions, and the mud—microcrystalline ooids are representative ooids in a relatively quiet environment. The broken and regenerated ooids may be in tight relation with storm. Although the oolids are totally dolomitized, the characteristics of cortex were preserved very well, it indicated that these oolids were mimetic dolomitized. Thus, the ooids in the study area may archived the primitive water chemistry of the sea in the Dengying period，and are the precious materials to reconstruct paleoseawater chemistry in Precambrian. The concentric and radial—concentric cortices implied that there might be aragonite sea during the Dengying period.