This paper reported the petrology, geochemistry and possible origin of the upper Qigebulake Formation (Upper Sinian) algal dolomite at Kalpin Area, Tarim Basin. Petrologically, matrix dolomite (MD), bladed cement dolomite (BCD), and rhombic cement dolomite (RCD) are recognized under polarizing microscope examination. MDs were euhedral micrite dolomites with crystal size ranges from 2－5 μm; microbe associated spheroidal and sheet-like dolomites are common. They lack any evidence of precursor minerals. BCDs are composed of a series of euhedral ultralmicro-dolomites, which parallel the c axis. The length of BCDs varies from tens of μm to 100 um. RCDs occur in the center of the pores with a crystal size less than 50 μm. Geochemically, MDs have the highest Na2O content (average 380 ppm), moderate FeO, MnO content (average 727 ppm and 175 ppm, respectively). BCDs have moderate Na2O content and lowest FeO content (average 290ppm and 200 ppm respectively); the MnO content is below the detection limits. RCDs have the lowest Na2O content (average 200 ppm) and highest FeO, MnO content (average 750 ppm and 550 ppm, respectively). The 13C values (2.1－3.0 ‰, PDB) are in agreement with the reported results; the δ18O values are relatively higher than those reported values of Paleozoic carbonates, which implies Sinian seawater is relatively rich in 18O. It is interpreted that the MDs are microbially mediated primary precipitates and do not develop by dolomitization of precursor carbonates. RCDs origin from the volume-per-volume dolomitization of aragonite cement by the mixing water of meteoric water and seawater; while RCDs form under burial environment.