The oncoid is a variety of microbial carbonate with double attributes: (1) amongst the coated grains the oncoid is distinguished from the ooid by its relatively large size and non- smooth lamina within its cortex; (2) it is a variety of spherical stromatolite with an obvious biosedimentation structure resulting from successive lamination processes. High- energy and micrite- dominated oncoids are developed within a massive ooid- grained bank in the Zhangxia Formation of the Cambrian Miaolingian at the Sandaogou section of the Huludao City in the western part of Liaoning province. Following features characterize these oncoids: (i) centimeter level size; (ii) chiefly oval morphology; (iii) the characteristic laminated fabric within the cortex; (iv) the common and relatively high- density preservation of calcified- sheath fossils of the filamental cyanobacteria within the cortex. Although the greatest challenge that remains is the interpretation of ancient oncoids and the details of their formation through the filter of the diagenesis, even the processes involved in calcification of biofilms dominated by the cyanobacteria are not well understood. The rich evidence represented by microbial fossils, especially for sheath fossils of the cyanobacteria within oncoids, in the top part of Zhangxia Formations at the Sandaogou section provides a good example of oncoids built by photosynthetic biofilms. They also provide an opportunity to understand the differences between processes forming the ooid within the water column and the oncoid growing on the seafloor.