Abstract: Nanometer TiO₂ photosemiconductor is a new kind of environmental friendly material, which has been widely used in environmental cleaning and antibacterial fields. However, there are still no reports on application of nanometer TiO₂ photosemiconductor in preventing plant diseases. This paper firstly reported the effects of TiO₂ sol in preventing Pseudoperonospora cubensis. The experiments showed that spraying TiO₂ sol significantly reduced disease indexes, disease incidence, lesion area and amount in cucumber leaves which indicated that TiO₂ sol effectively inhibited the development of Pseudoperonospora cubensis. These results provide some scientific references for the development of environmetal friendly antibacterial and antifungal agents using nanometer TiO₂ photosemiconductor as effective intergradient in preventing plant diseases.