Pathogen of tomato powdery mildew in Yinchuan of Ningxia was isolated and purified, then characterized by morphology and molecular biology methods. Morphology of the pathogen showed that its conidia were solitary, oval or drum shaped; germ tubes germinated at the top of conidia, mastoid appressoriums formed at the end of germ tubes which also occurred on hyphae; conidiophores were erect and unbranched; no closed capsule was found. Internal transcribed spacer sequence alignment showed the pathogen have more than 99% consistency with the Oidium neolycopersici. Morphological and molecular biological identification showed that the pathogen causing tomato powdery mildew in Yinchuan was Oidium neolycopersici.