The effect of six SiO ₂ nanosized concentrations (0, 5, 20, 40, 60 and 80 mg L ^-1 ) and three seed prechilling treatments (control, seed prechilling before nano SiO ₂ treatments, treatments of seed with nano SiO ₂ before prechilling) on germination and seedling growth of tall wheatgrass (Agropyron elongatum L.) were studied. Results indicated that application of SiO ₂ nanoparticles significantly increased seed germination of tall wheatgrass from 58 percent in control group to 86.3 and 85.7 percent in 40 and 60 mg L ^-1 , respectively. Applying SiO ₂ nanoparticles increased dry weight of shoot, root and seedling of tall wheatgrass. Increasing concentration of nanoparticle from 0 up to 40 mg L ^-1 increased seedling weight around 49 percent compared to the control, nevertheless decreased under 60 and 80 mg L ^-1 treatments. In conclusion, seed prechilling in combination with SiO ₂ nanoparticles largely broke the seed dormancy for A. elongatum.