Abstract: The Ordovician Majiagou Formation in Sulige region of the Ordos Basin was deposited in hypersaline restricted epicontinental environment.It is mainly composed of carbonate rocks with thin evaporate interbeds.The dolomites are classified into 3 kinds:micrite,fine-medium crystal dolomite,and dolomite filled in pores and fractures.The Na concentrations range from 0 to 2 099×10^-6,the Sr concentrations range from 0 to 70×10^-6,the Mn concentrations range from 0 to 112×10^-6,and the Fe concentrations range from 39 to 86 746×10^-6 in dolomites.The dolomites are characterized with varied Na,low Sr and Mn,and high Fe concentrations.The ¹⁸O values for dolomites range from-16.00‰ to-5.73‰,and the ¹³C values range from -11.46‰ to 1.90‰(PDB).It indicates that carbonate rocks have experienced leaching and burial diagenesis.Most filled dolomites have lower ¹⁸O and ¹³C values than surrounding components,indicating that the former was formed later.The dolomites are characterized with low degrees of order and less stoichiometry.The ⁸⁷Sr/⁸⁶Sr values of dolomites range from 0.707 977±0.000 070 to 0.711 791±0.000 064,most of which are higher than that corresponding to global boundary between middle and upper ordovician.Microbial dolomitization is the reason for dolomite genesis.