A small amount of dolomitization is found in the Upper Ordovician Lianglitage Reef -Shoal complex reservoir of Tazhong-Ⅰ Slop-Break Zone in Tarim Basin. The dolomitization commonly are in three fabics: (a)the calcic vadose silt of cave filling, (b) mud matrix of bioclastic packstone, and (c) calcareous alga, eophyton sand and alga-mud sand of grainstone. The result of the dolomitization directly contributes an average 1 percent of the porosity to the reservoir, while the intercrystal pores provide for channels of further erosion, which plays an important role to the formation of an effective reservoir. Biological high-Mg calcite is the important source of magnesium ions for dolomitization. In-depth study on the distribution of Crinoid and Calcareous Alga rock is required to predict this dolomitization-associated reef-shoal reservoir.