Abstract: Disputes have occurred for a long time on the forming mechanism of the dolomite in the Lower Ordovician Penglaiba Formation in the Gucheng area of Tadong low uplift, Tarim Basin, that have seriously hindered oil and gas exploration there. This time, based on the observation of cores, thin sections and cathodoluminescence results, the geochemical characteristics of rare earth elements from various forms of dolomite, such as sparite, cave filling and micrite, are systematically analyzed. All the testing results are standardized by the rare earth elements in micrite. There observed three types of REE distribution patterns which suggest three kinds. The first kind of dolomitization is the buried dolomitization by fluids; the second kind of dolomitization formed in a hydrothermal environment, and the third kind driven by evaporative pump. Some dolomite formed earlier may be further modified and/or reformed by other kinds of dolomitization fluids later on.