Abstract: Wumishan Formation of middle Proterozoic in Jixian county, Tianjin, mainly consist of bio-dolostone, about 3000m in thickness. As the results of field section survey, microscopic analysis of thin sections and electronmicroocopic scanner of rock deperis, the whole process of development and evolution of oncolite is recognized. The dispersal grain is the embryonic form of oncolite, its different pattern and character of coat reflect various occurence in the early stage of coated grain. If we regard the whole process of growth from initial dispersal grains to resultant product of oncolite as a series of processes, its growth envi-ronment is not confined to conventional recognization-sedimentary medium is a shaking and low-energy state. Vertical sequence of Wumishan Formation show spiral-rhythm accumlation and within each sequence algal coat from dispersal grain to oncolite also display synchronous evolution rhythm. Diameter of grams varies from 0.5-2mm to 10-20mm. Periodical and sprial evolution series of algal coat arc from simple to com plex. Macroscopically, disperal grains are black-brown spherical grains, containing much organic materials. They float and disperse in the dolostone. Under polarization microsoope. they are radiation concentric, winkle pisolit and flat leaf. Under electronic microscope, a flower is formed by a few slices of dolostone gathered together, with diameter of several to tens of μm.