Abstract: Abstract：Karstification is one of the most important elements for the formation of Ordovician marine carbonate reservoirs in northern Tarim Basin. Based on lithology, geophysical data and test data, the authors divided karstification of northern Tarim Basin into three types (penecontemporaneous karstification, buried karstification, weathering karstification) and some subtypes. The penecontemporaneous karstification controls the formation and distribution of the early carbonate rock reservoirs, the buried karstification is always developed along the former pore-fracture systems, and constitutes one of the most key factors for reservoir optimization, and the weathering karstification is the key factor for the formation of Ordovician carbonate reservoirs. As for the characteristics of polycyclic structure evolution of northern Tarim Basin, there exist six periods of paleo-karstification in different superimposing and rebuilding stages and various Ordovician carbonate rocks, of which the most important periods for reservoir karstification are the early Caledonian II episode, the middle Caledonian I and II episode, and the early Hercynian. Different types of karstification in northern Tarim Basin have complex and pronounced superimposition relationship： the penecontemporaneous karstification provided basic pathway for the buried karstification； the subsequent buried karstification and weathering karstification were developed on the basis of succeeding to and superimposing on the former penecontemporaneous karstification and finally became the excellent potential reservoir.