Mutual　transformations　of　various　nitrogen　compounds　and　their　reaction　mechanisms　have　been　a　subject　of　great　concern　to　the　chemical,　ecological　and　environmental　communities.　In　the　paper,　the　reactions　of　NO3-　ion　with　small　organic　acids　such　as　formic　acid　(HCOOH),　acetic　acid　(CH3COOH)　and　lactic　acid　(C3H6O3)　under　ultraviolet　illumination　were　investigated　systematically.　It　was　found　that　NO3-　ion　is　easily　reduced　into　NO2-　and　NOx　and　then　further　into　N-2　and　NH3　(in　the　form　of　NH4+)　in　the　process.　The　carboxyl　anion　radicals　and　hydrogen　formed　by　photodecomposition　of　formic　acid　are　responsible　for　the　rapid　photoreduction　reaction　of　nitrate.　The　initial　pH　and　the　nitrate　concentration　considerably　affect　the　product　distribution　and　nitrate　conversion.　Based　on　a　preliminary　simulation　study,　we　speculated　that　the　photoinduced　reaction　may　effectively　proceed　in　oceans,　lakes　and　rivers　because　of　ever-increasing　nitrate　and　organic　emissions.　This　research　is　helpful　to　understand　nitrogen　cycle　mechanism　and　develop　water　environmental　control　technologies.