Abstract: The air anion content of Cunninghamia lanceolata, Quercus acutissima, Phyllostachys pubescens, Pinus massoniana were observered in Nanjing. The results showed that daily variation of the content of air anion had only a singlepeak in C. lanceolata and Ph. pubescens, and a double-peak in Q. acutissima, however the difference of the daily variation of the air anion in the four forest stands were insignificant. The monthly variation of air anion in the four forest stands were similar and performed a single-peak each. Variation of the air anion contents of different seasons in the four forest stands were in a descending order of summer, autumn, spring and winter, the difference was significant. The average content of air anion in C. lanceolata, P. massoniana, Q. acutissima, Ph. pubescens were 1 234, 1 387, 1 157, 1 198 ions/cm³ respectively. The most important factor that affected the content of air anion was crown density, followed by productivity and forest vertical structure.