Background: TOXOPLASMA GONDII, the coccidian protozoan parasite with worldwide distribution, is the agent of toxoplasmosis. The disease is life threatening in congenital form and in immunocompromised patients. The present study was carried out in 2016 to evaluate the in vitro effects of nanosilver colloid on tachyzoites and bradyzoites of T. GONDII, RH and Tehran strains. Methods: Different concentrations (5, 10, 20 ppm) of nanosilver colloid were added to tachyzoites of T. GONDII, RH strain (type I) and bradyzoites and tissue cysts of T. GONDII, Tehran strain (type II) and incubated for 30, 60, 90 and 120 minutes. The mortality rates of tachyzoites and bradyzoites were evaluated by trypan blue dye and MTT assay. Then SEM carried out to show the changes between control and exposed parasites. Results: The greatest mortality rate was seen in 20 ppm concentration and after 120 minutes of exposure. By electron microscopy, the structural changes were seen in tachyzoites of RH and tissue cyst of Tehran strain in comparison with control groups. Conclusion: Nanosilver colloid was effective on both tachyzoites and bradyzoites of T. GONDII, RH and Tehran strains.