Abstract: Late Ordovician organic reefs composed of cyanophyta and calcareous sponge were firstly discovered in northern Tarim basin.Study showed that they can be classified into algal binding calcisiltic buildup reef and calcisponge-cyanophyta baffling-binding buildup reef.From study of paleoecology of reeef body, recognition of cyanophyta and calcisponge community was made, and their evolution was discussed.The study on their origin and mechanism indicated that the reefs in study area were formed resulted from co-operating of organisms and water power, namely co-operating of algal binding, calcisponge baffling and mechanical depositing, and went through following four stages: colonization, binding, baffling-binding and declination, their evolution was distinctly controlled by the environment.

Key words: Tarim basin, Late Ordovician, Organic reef