The Day river basin located in the delta region of the Red River in the Nord of Vietnam is now sufferingstrong deterioration of water quality due to rapid population growth, industrialization and economicdevelopment. Nitrogen isotopes (δ15N and δ18O) have been used to identify N sources and processes inhundreds of studies over the past several decades. The isotope approach is based on the fact thatnitrates from different origins have distinct isotopic signature ranges. Inorganic nitrate fertilizers showsignificantly higher δ18O values compared to most other nitrate sources, whereas their δ15N value isgenerally quite low. In contrast, nitrate derived from organic sources tends to exhibit elevated δ15Nvalues, but comparatively low δ18O values. This research aims to elucidate the source of pollution bynitrate in aquatic environment based on the isotope analysis. The analysis of nutrient content in waterhas been carried out through the present of δ15N and δ18O concentration. The high percentages of theseforms suggest the major sources of nitrate may derive from manure and septic waste, fertilizer andprecipitate of nutrition in surface water.Key words: isotope analysis, sources of nitrate, δ15N and δ18O concentration, Day river