Marine nitrogen cycle plays a key role in marine biogeochemical cycles, which has its impacts on the structure and function of regional and global marine ecosystem, and is closely related to global climate change. Nitrogen and oxygen isotopes of nitrate reflect the major nitrogen transformation processes in the ocean, and become important tools in the study of marine nitrogen cycle. In this paper, we summarized recent progresses in the analysis of nitrogen and oxygen isotopes of nitrate in seawater, the isotopic fractionations during the major nitrogen cycling processes, i.e. assimilation of nitrogen nutrients, nitrification, denitrification, dinitrogen fixation, and their application in marine nitrogen studies. The distribution of nitrogen and oxygen isotopes of nitrate within marine ecosystems can provide a record of the sources of nitrogen supporting biological production and the major pathways and mechanisms moving nitrogen through different pools. Current challenge for successful use of nitrogen and oxygen isotopes of nitrate in resolving nitrogen cycle fluxes and processes requires an analytical advance, an accurate construction of nitrogen budget, and a further understanding of processes affecting nitrogen and oxygen isotopes in the upper ocean. The spatial and temporal variation in the isotopic composition of nitrate requires a sampling extension among global oceans, especially in China marginal seas with little data now.