Origanum vulgare was used as an effective herbal for infertility treatment in traditional medicine. The present study compared the effects of the hydroalcoholic extract of the air branch of Origanum vulgare, LHRH-A_2, and17β-estradiol on the ultrastructure of oocyte and pituitary in immature Trichogaster trichopterus. For this purpose 60 pieces of Trichogaster trichopterus with average weight of 2.1±1gram were divided into 6 groups: saline, placebo, ethanol and 3 experimental groups (Origanum vulgare and 17β-estradiol at a dose of 50 mg/kg and LHRH-A₂ at a dose of 0.005 mg/kg) (each group 10 fish). Fish were injected intra-muscularly, one dose every other day and seven doses in total. Ultrastructure of oocyte and anterior pituitary in the three treatments were measured and compared with control treatments. The results showed that the size of oocytes follicle and the mean diameter of Golgi vesicles in Origanum vulgare treatment was higher than that in LHRH-A₂ treatment, but there were lesser than that in 17beta-estradiol treatment (p≤0.05). The results of the ultrastructural comparision of the anterior pituitary showed that in Origanum vulgare treatment the number of small cells increased and made a few large granules in gonadotroph cells. In fish treated with 17β-estradiol the number of large granules increased significantly, whereas in LHRH-A₂ treatment large granules were lysed and stimulation was complete. The present results suggested that the Origanum vulgare affected Trichogaster trichopterus fertility.