Abstract: The Mianhuakeng uranium deposit is a large granite-type uranium deposit in Southern China. The carbonatization is closely correlated with the uranium mineralization in this deposit. The calcite of different stages of deep seated orebodies in the Mianhuakeng uranium deposit were sampled and the carbon and oxygen isotopes were investigated. The calcite veins in the Mianhuakeng uranium deposit were divided into three stages, namely pre-mineralization stage, the principal mineralization stage and post mineralization stage. The values of δ¹³C_PDB of calcites decreased from the pre-mineralization to the principal mineralization to the post-mineralization stages. The value of δ¹⁸O_SMOW was the smallest in the pre-mineralization stage, and was similar with the average value in the principal and post-mineralization stages. The δ¹³C_PDB of calcites was from -4.9‰ to -6.0‰ with -5.5‰ in average in the pre-mineralization stage, which were consistent with CO₂ in the mantle. The δ¹³C_PDB decreased in the principal mineralization stage and post-mineralization stage, because of the isotope fractionation during the degassing of CO₂, indicating that the carbon of ore-forming fluid of Mianhuakeng uranium deposits was mainly from mantle. The δ¹³C_PDB performed the negative correlation with the δ¹⁸O_SMOW, suggesting that the mineralizer in ore-forming fluids of Mianhuakeng uranium deposit was formed by the mantle degassing.