Three types of paleokarstification, including syndepositional, weathering-crust, and buried ones, are distinguished in Ordovician marine carbonate within Tarim Basin. It is synthetically considered that the paleokarstification is a key factor of the formation of Ordovician carbonate reservoirs. Intragranular and intergranular solution spaces and moldic pores formed by syndepositional meteoric fresh-water selective dissolution are provided matrix porosity for the carbonate reservoir.