Carbon isotopes(δ¹³C) of carbonate from sediment core has been measured in Lake Caohai, Guizhou Province, south-west China.The results showed that δ¹³C values varied between -14.25‰ and 23.10‰, which was the largest amplitude of car-bon isotope variations and the highest positive values discovered so far in lacusrine carbonate sediments.On the basis of combina-tion with oxygen isotope, organic matter and carbonate content proxies, we suggested that the carbon isotope of carbonate from Lake Caohai was controlled by photosynthesis/respiration of aquatic plants, and the input of bacteria to carbon isotope fractionation of or-ganic matter might be responsible for the abnormal positive δ¹³C values in Lake Caohai.Therefore, much attention should be paid to the carbonate δ¹³C in Lake Caohai in the future research.