Abstract: The disease of powdery mildew on Platanus sp.in Nanjing is comparatively serious development in recent years. Morphological observation of anamorph and molecular biological identification of teleomorph of this pathogenic were included in this paper. The result showed that anamorph of the pathogenic was identified as Oidium sp., teleomorph was Erysiphe platani. The suitable temperature for the pathogenic growing was from 20 to 28 ℃, and 26 ℃ was the most suitable growth temperature. The effect of humidity was not significant,conidia could germinate in general environmental humidity. The life of conidia was about 13 days.