Denitrification plays an important role in wastewater treatment systems for the reason that the nitrate or nitrite was reduced, and some gases such as NO, N₂O or N₂ were released. The application of denitrification in wastewater, the mechanism of denitrification, and the effect factors of denitrification were introduced in this paper. The communities of Denitrifying bacteria, and some key enzymes of denitrification were also introduced. The discovery of aerobic denitrifying bacteria, autotrophic denitrifying bacteria, and denitrifying Phosphorus-removing Bacteria were also mentioned in this paper.