The breeding ecology of Common Tern (Sterna hirundo) and Gull-billed Tern (Gelochelidon nilotica) were studied in Hongjiannao Lake of Shaanxi Province from April to July of 2009 by direct observation and by counting the nests. Both of them arrived at Hongjiannao Lake in late April and bred in the middle of May. Both species built their hest at the islands in Hongjiannao Lake,but the nest-sites of S. hirundo located in the margin area of the islands in linear patterns,and the nest-sites of G. nilotica were built in the center region in a patch patterns. Food items of the two species were partially separated. Therefore,the separations in nest-sites and food items were regarded as the major reasons for the two identical bird breeding in the same area. The growth curves of the nestling body mass of the two species matched with the description of Gompertz equation and the difference in growth rate of the two birds might reduce the interspecies competition.