Nitrate is a highly concerned pollutant in global aquatic ecosystem, resulting in eutrophication and water quality deterioration. As a result of the dissolved inorganic carbon fertilization effect, lake ecosystem is expected to respond more sensitively to nitrogen (N) addition in karst region than in non-karst region. Identifying accurately the sources of nitrate in lake system is an important prerequisite for formulating effective strategies on reducing nitrate and restoring water ecosystem. Quantitative identification of nitrate sources to lakes in karst region is limited until now. In this study, delta N-15-NO3- and delta O-18-NO3- were jointly used to identify the nitrate sources in Caohai Lake, a typical karst lake. The delta N-15-NO3- and delta O-18-NO3- of lake water displayed significant seasonal variations. The average delta N-15-NO3- values during normal, dry and wet seasons were 6.6 parts per thousand, 12.7 parts per thousand and 0.9 parts per thousand, respectively. Accordingly, the average delta O-18-NO3(-) values were 11 parts per thousand, 13 parts per thousand and 16 parts per thousand, respectively. The average contribution percentages of nitrate from agricultural activities, precipitation and sewage were 42 %, 41 % and 17 %, respectively. Strict measures should be taken to prohibit unreasonable agricultural activities and to improve nutrient use efficiency through optimized fertilization and irrigation management. In view of the sensitivity of karst lake ecosystem to N/P addition, higher discharge standard requirement is necessary for restoring water ecosystem and maintaining good water quality. This study proved that the combined use of delta N-15-NO3 and delta O-18-NO3- is a promising method for identifying quantitatively the nitrate sources in karst lake system.