As a particular variety of microbial carbonates, the oncoid is characteristic of double natures as following: ① the nature of the coated grain demonstrated by both the relative size and the non- smooth cortex, which common means its microbial origin and is distinct from the ooid whose origin remains uncertain; ② the nature of the stromatolite expressing as a type spherical or ellipsoid biosedimentary structure resulted from successive lamination process around the nuclei, which is a fundamental feature that is used as a basis to group the oncoid as a spherical stromatolite. One set of particular cap oncolitic grainstones of bank oolitic grainstones with the thickness of about 1 m was developed in the top part of oolitic grainstones of the carbonate bank belonging to the upper part of the Zhangxia Formation of the Cambrian Miaolingian at the Chafangzi section in Fanshi county of Shanxi procince, which represent a spectacularly sedimentological phenomenon. Many features characterize these oncoids within the cap of oncoidal limestones with the relative small thickness (about 1 m): ① the centimeter- level size ( 1 to 3 cm); ② the morphological feature of sphericity or ellipsoid; ③ both the dominated micritic and the secondary microspar composition of the oncoidal cortex; ④ the highly dense preservation of filamental sheath fossils of the cyanobecteria within oncoidal cortex; ⑤ a special matrix of clotted micrites with clots that are composed by filamental sheath fossils of the cyanobecteria. Although the greatest challenge that remains is the interpretation of ancient oncoids and their detail forming processes through the filter of the diagenesis, also the detail of the calcification of biofilms dominated by the cyanobacteria has still not get better understanding, the richness of direct evidence represented by microbial fossils especially for sheath fossils of the cyanobacteria within oncoids of a particular oncoidal cap in the top part of the Zhangxia Formations of the Cambrian Miaolingian in Fanshi county of Shanxi province make them become a good example of oncoids built by non- layer photosynthetic biofilms, which is an example for the further understanding of “algale flat” deposits developed on the oolitic- grained bank.