Abstract: The ecological structure link between mangrove ecosystem and aquatic food production is of great importantly for fisheries management. The objectives of the research to know well about are to :(1) determine the stable C and N isotopic compositions of fish species and mangrove leaf (2) use bulk and compound-specific isotopic ratios to determine relative tropic positions for fish species in mangrove ecosystem of MuaraBadak coastal area – East Kalimantan, Indonesia. A few of fish species found in mangrove ecosystem was examined on the basis of carbon and nitrogen stable isotope distributions. Result of the study revealed that significant differences in stable isotope signatures (C and N) exist in some species from different habitats in that coastal area, which could be used to delineate feeding habitats of fishes. The space mangrove area and dense mangrove area do not appear to be direct sources of carbon in the diets of studied fish species; rather, they probably serve as refuge as well as a substratum for a variety of primary producers and consumers that are important in the food webs of these habitats. All fishes showed higher δ15N values suggesting with a higher trophic level. Some fish species of similar feeding guilds showed some degree of segregation by feeding on different food resources. The food web in the mangrove ecosystem of this coastal area showed various trophic levels. The fishes were the most enriched in δ15N. Also, clear gradients in both δ13C and δ15N could be observed for different feeding guilds of fishes and for different habitats such as mangrove area and rivermouth of this coastal area. Based on analysis of stable isotope ratio (both δ13C and δ 15N) of fish species showed that the mangrove plant have basically food chain on mangrove ecosystem. Nevertheless, the biodiversity and community structure of mangrove had been decreasing presently in MuaraBadak coastal area, it that reason to keep and to save mangroves ecosystem and also promote the sustainable use management of mangrove ecosystem.