The effects of nano-size titanium dioxide ($N-TiO_2$) on the germination of tomato (Lycopersicum esculentum L.), onion (Allium cepa L.), and radish (Raphanus sativus L.) seeds were assessed in laboratory and greenhouse trials. Seeds were germinated in Petri dishes in a laboratory and in peat:perlite (1:1, v/v) in a greenhouse containing four concentrations of $N-TiO_2$ (0, 100, 200, and $400mg;L^{-1}$). $N-TiO_2$ at 100 and $200mg;L^{-1}$ had the most positive effect on germination. In the laboratory, the highest germination percentage of tomato and onion was observed at $100mg;L^{-1}$ (100 and 30%, respectively), and in radish, 100% germination was obtained with $400mg;L^{-1}$. In the greenhouse, seedlings were tallest after exposure to 400 and $200mg;L^{-1}$ for tomato and onion, respectively, and 400 and $100mg;L^{-1}$ for radish. $N-TiO_2$ may serve as a seed-priming agent for horticultural crops.