Rice blast disease, caused by Magnaporthe grisea, is the most serious biotic threat to rice (Oryza sativa L.) production worldwide. It causes severe yield losses in Egypt especially in epidemic years. The fungus is highly variable so disease control is a challenge. In this study, the effect of silver nanoparticles (20-30 nm) against rice leaf blast fungus was evaluated under different cultivation conditions both in vitro and in vivo. Under lab conditions, the application of four concentrations of silver nanoparticles to the culture of M. grisea showed significant inhibition of both hyphal growth and number of colonies formed in a dose-dependent manner. Number of spores/ ml decreased with in all treatment. Under greenhouse conditions, Silver nanoparticles were sprayed in concentrations 0, 25, 50, 100 and 200 ppm on rice seedling leaves at three times (3 hours before inoculation, 1 and 5 days after artificial inoculation with spore suspension). Damaged Leaf Area Percentage (DLA %) indicated that the application of 100 ppm silver nanoparticles was highly efficient before and after inoculation (26.7, 15.3 and 20%), respectively compared to the untreated plants of 80%. The chemical fungicides isoprothiolane (Fuji one) and azoxystrobin (Amistar) at a concentration of 100 ppm each showed the lowest DLA (19.6 % and 14.7 %, respectively). Scanning electron microscope results revealed that the silver nanoparticles caused a detrimental effect on mycelial growth.