Abstract: Fractures created by tectonism are one of the most important elements for the formation and rebuilding of Ordovician carbonate karst reservoirs in the northern Tarim Basin. Based on a number of comprehensive studies on cores and thin sections by cathode luminescence, FMI and testing, at least three sorts of diagenetic fractures and three stages of structural and dissolved fractures were recognized in Ordovician limestones of the northern Tarim Basin. The three stages of structural fractures corresponded to three phases of tectonic cycles, i.e. the late Caledonian to early Hercynian structural tension fissure filled with calcite and argillaceous fillings, the late Hercynian to Indo-China conjugated shear fracture filled with calcite and the Yanshanian to Himalayan shear network micro-fracture half-filled or unfilled. The formation of structural fractures directly accelerated karstification, and the dissolved fracture commonly reworked the former structural fracture, which occurred a little bit later though they belong to the same stage. The formation time and scale for various fractures as well as their filling characters and characters of FMI and cathode luminescence were different. Structural fractures and dissolved fractures controlled the development and distribution of karst reservoirs. Various karst-fractured reservoirs made up of fractures that were dissolved and enlarged or linked with solution pores became the major space for oil-gas accumulations, thus, these fractures are one of the most important elements for the formation and rebuilding of the Ordovician marine carbonate reservoirs. While the diagenetic fracture, despite of accelerating kastification somewhat, might have made small contributions to karstification due to less splaying and restricted horizontal connections for subsurface fluid transportation.