Nitrogen isotopic technique was used to study the nitrate pollution of four inflowing rivers of Lake Chaohu in different seasons. Our results showed that River Shiwulihe was the most polluted river, while River Shuangqiaohe as the least polluted one. Nitrogen Stable isotopic values demonstrat that nitrate sources differ in various seasons for those four rivers. The dominate pollution sources of nitrate for River Shiwulihe and River Nanfeihe are human sewage and industry discharge. In the case of River Paihe, nitrate is mainly derived from industry discharge in winter, which is originated from agriculture and rainfall in spring and summer, respectively. For River Shuangqiaohe, nitrate contamination is mainly caused by agriculture in winter and spring, but is influenced by rainfall in summer. In addition, our results imply that ammonium is the main nitrogen pollution compound in those four rivers, and δ¹⁵ NH₄⁺ value should be detected for the identification of ammonium sources in further study.