In order to investigate nitrogen provenance in different ecotype lakes (natural or urban), samples of water, surface sediments, hydrophytes and zoobenthos in lakes were collected, and their isotopic compositions of carbon and nitrogen were measured. The results indicated that δ¹⁵N-NH₄⁺ and δ¹⁵N-NO₃^- in the water of Lake Banghu were -1.8‰ ± 1.0‰ and -0.5‰ ± 1.7‰, respectively, which showed the major inorganic nitrogen from rainwater and farming fertilizer. The δ¹⁵N-NH₄⁺ in Lake Xianghu water was high (6.8‰ ± 8.6‰), especially in fish wastewater (13.5‰) and sewage outlet (25.4‰), which showed sewage input. The δ¹⁵N-NO₃^- was partially negative (-2.9‰ ± 4.2‰) and showed that the nitrate nitrogen was not mainly from anthropogenic input. The δ¹⁵N of organic matters was not significantly different from that of surface sediments and hydrophyte (6.6‰ ± 0.3‰ and 7.1‰ ± 0.7‰, respectively) in Lake Banghu, so hydrophyte was the major source of organic matter for sediments. In Lake Xianghu, there were wide distributions of δ¹⁵N, δ¹³C and organic C/N in in the organic matters of sediments (δ¹⁵N:3.6‰-8.3‰, δ¹³C:-27.1‰--24.7‰, organic C/N:2.6-10.8, respectively), which implies the complexity of the sources of organic matter provenance in the urban lakes.