[Objective] The effects of nano-TiO₂ semiconductors sol (nano-TiO₂ sol) on plant diseases prevention were studied. [Methods] The antibacterial abilities of nano-TiO₂ sol were identified by using bacterial pathogens of Pseudomonos syringae pv. lachrymans and Xanthomonas vesicatoria. The effects of nano-TiO₂ sol on plant diseases prevention were measured by method of artificial inoculation experiment with cucumber plants inoculated with Pseudomonos syringae pv. lachrymans and field experiment with cucumber seedling naturally infected by  Pseudoperonospora cubensis. [Results] Results showed that nano-TiO₂ sol possessed strong antibacterial power by forming continuous and stable antibacterial films on surface of substances. Artificial inoculation experiment and field experiment indicated that spraying cucumber leaves with certain concentration of nano-TiO₂ sol could significantly reduce lesion areas, disease incidences and disease indexes. [Conclusion] It was initially confirmed that nano-TiO₂ sol could effectively inhibit the development and pervasion of plant bacterial / fungal diseases.