Abstract: Algal and nonalgal dolomites develop extensively in Sinian System, Southwest of Sichuan Basin, which arc good natural gas reservoir beds. Some experts have done much basic research on their origin, but they had got dif ferents conclusions. Based on the previous works and the combinations of global and reginal tectonizations with advanced laboratory analyses, the medium conditions and origin of dolomites, have been deeply analysed, it was proposed that Sinian System dolomites consiste of primary dolomites and secondary dolomites. It is the first time that the unit cell parameters and crystalinity have been used for distinguishing primary dolomites from secondary dolomites. The developing of algal dolomites, microlite and micritic dolomites were controlled by the factors such as global tectonic settings, sea level changing, hot palaeoclimate, evaprat tidal flat environment, strong and weak of water dynamics and so on. These dolomites were percipitated in marine tidal enviroment by trapping, cohering, and biochemical processes of blue-green algals. They have lower crystallinity from 0.682 to 0.744 with average of 0.696, higher salinity, higher contents of trace elements (Sr, Ba, Sb, Sn, Cu) , and Mg/Ca ratio gererally lower than 0.5. The a。 and c。 of their unit cells are smaller than those of standard dolomites, its crystall is fine and has a low automorphic degree. Pure fine-crystall dolomites were developed by dolomitization under the environment that salty enviroment have had changed by air-water in early diagenetic stage. They constitute of rhythmic structure, the colour of cathodoluminesence consist of yellowish red and dark red. Fine-crystall quartz contained dolomites with dark red cathodoluminesence colour were fomred in late diagenetic stage of deeply burial enviromnt by dolomitization, residual quartz is wide spreaded in it, crystallinity of this kind of dolomite is higher (0.896-0.992) and its δ~(18)Ois light (-8.64-9.01‰) with the Mg/Ca greater than 0.5, its unit cell a。 is 4.876A and c。 is 15.249A which are similar to those of standard dolomite, and its crystall is croase, possesing a high automorphic degree.