Abstract: We discussed the composition and factors influencing carbon and oxygen isotopes in dolomites and coexisting calcites in hand specimen mesoscale based on petrography and combined with elemental, X-ray diffraction and carbon and oxygen isotope analyses. Results indicated that dolomites have higher δ¹³C and δ¹⁸O values than coexisting calcites. Two factors may account for the higher δ¹⁸O values of dolomites. (1) The oxygen isotope fractionation factors of a dolomite-water system are higher than those of a calcite-water system. Therefore, oxygen isotope fractionation factors are greater than 1 in a dolomite-calcite system. (2) The fluids of dolomite precipitation have higher salinity and higher ¹⁸O abundance compared to coexisting calcite. In addition, the δ¹³C values of dolomites show the same trends with δ¹⁸O values. That is to say, the carbon isotope fractionation factors in a dolomite-CO₂ system are higher than those in a calcite-CO₂ system. Therefore, carbon isotope fractionation factors are greater than 1 in a dolomite-calcite system during replacement process. Oxygen isotope values varied obviously between syndepositional dolomites and coexisting calcites, while they were similar between hydrothermal/deeply burial dolomites and coexisting calcites.