Trichomoniasis is one of the common diseases in birds and poultries which is caused by flagellate protozoa called Trichomonas. Today, some synthetic drugs are used such as Metronidazole which has some side effects and drug resistance but in past years some medical herbs such as fenugreek, mint, garlic and etc. were commonly used. In this research, the antiparasitic effect of garlic extract (Allium sativum) on Trichomonas gallinae in comparison with Metronidazole has been studied. In this experimental study, the plants were collected, dried. The ethanol extract was prepared and samples were assessed by HPLC. 106 liquid Trichomonas trophozoite/ml was added to the  diamond medium. The three groups were culture tube containing 0.1 mL Metronidazole and Trichomonas gallinae, Trichomonas gallinae plus 0.1 mL saline as a control group and  tubes containing concentrations of  0.1, 0.01 and 0.001% ethanol extract of garlic  in a solvent of nanopure distilled water and Trichomonas gallinae (5 tubes for each concentration). All tubes at zero time, containing 106 live trophozoites alive and were counted with 15 minutes interval in the first two hours and then at hour 3, 4, 5, 6, 12 and 24. The results revealed that all concentrations of garlic extract werean antiparasitic effect on Trichomonas but 0.01% concentration was the most effective as similar as metronidazole. In these two tubes, all Trichomonas protozoa were not alive 260 minutes after the onset of  the study. The according to  results and other researches show that garlic extract can be effective on some parasites and can be used to treat some parasitic diseases.