The partial pressure of carbon dioxide p(CO2) in the surface water of the two karst eutrophic reservoirs, Hongfeng and Baihua Lakes near Guiyang city, Ghuizhou Province, China has been studied for one year. The factors regulating the seasonal variability of p(CO2) in the two reservoirs have been analyzed. Unlike the reservoirs in boreal temperate regions, the p (CO2) in the two reservoirs has significant seasonal changes: CO2 was unsaturated during summer, but supersaturated at other seasons. According to the correlation analysis, there is a negative relationship between p (CO2) and Chla in the two reservoirs, due to the production of the phytoplankton and the respiration of the organic matter by the bacteria, which are also the main factors regulating the p(CO2). The negative relationship between temperature and p(CO2) is considered to be due to the effect of temperature on the growth of the phytoplankton. Precipitation leads to the input of the nutriments into the reservoirs, and hence precipitation is negatively related to p(CO2). The relationships between NO3-, NO2- and p(CO2) are ascribed to the absorption of phytoplankton, and the degradation of the organic matter nitrification. The relationship between SiO3(2-) and p(CO2) is due to the absorption of phytoplankton and the precipitation. The differences of the relationship between p(CO2) and DOC in the studied two lakes possibly lie in the variations of their sources.