Artemisia annua L. is a well-known source of artemisinin, an antimalarial drug. This compound has been used in traditional medicine to treat malaria for many years. The anthelmintic property of A. annua has also been attributed to artemisinin, hence the belief that artemisinin could be useful as an alternative anthelmintic to control Haemonchus contortus in livestock. The present study focused on evaluating the anthelmintic activity of A. annua against H. contortus by oral administration on infected sheep in a controlled clinical trial. The trial was conducted using 24 male sheep of the Santa Inês breed, with average weight of 20 kg, artificially infected with H. contortus (Embrapa 2010 isolate). The animals were kept in metabolic cages and randomly divided into four homogeneous groups with six animals each, being: (T1) negative control (untreated), (T2) positive control: 10 mg/kg BW of levamisole phosphate, (T3) fed 10% A. annua (0.2% BW), (T4) fed 20% A. annua (0.4% BW). EPG counts were conducted on days -3, -2, -1, 0 (treatment day), 3, 7, 10, 14, 17, 21, 24 and 28 post-treatment. After extraction of the artemisinin in plant material, it was quantified in triplicate and analyzed by highperformance liquid chromatography with infrared detection. The average efficacy in the positive control treated with levamisole was 89%. Moderate anthelmintic efficacy against H. contortus was observed in the groups fed 0.2 and 0.4% BW A. annua for 30 days (32.9 and 14.8%, respectively), mainly from day 21 post treatment (47.1 and 25.2%, respectively). At the highest dose, the animals avoided eating due to the compound's bitter taste. It is concluded that A. annua presents moderate anthelmintic activity in sheep in both doses. The lowest dose was accepted by animals and seems to have practical use. Oral administration was safe and after further in vivo trials could be introduced in organic farms in tropical countries.