This study was designed to examine in vitro and in vivo anthelmintic efficacy of condensed tannins (CT) extracted from seeds of Medicago sativa on Haemonchus contortus in sheep. CT's in vitro anthelmintic effect was assessed at a 300 μg/ml concentration compared with albendazole (reference drug) at 10 μg/ml. The results showed that CT had a nematocidal effect on H. contortus, and the cuticle of the adult worm appeared to be its initial target. For the in vivo experiment, nine 3-month-old helminths-free lambs were distributed into three groups. Group 1 (n=3) was challenged only as infected untreated controls; Group 2 (n=3) was treated with condensed tannin, and Group 3 (n=3) was treated with albendazole. Fecal and blood samples were collected every 3 days until the end of the experiment; for fecal egg count (FEC) and anti-H. contortus IgG titers determination, respectively. The lambs treated with the CT in G2 exhibited a pronounced decrease of mean FEC with great FECR% detected from the first-week post-treatment (PT) until the end of the experiment compared with G1 animals. The antibody levels gradually increased in G2 following the 2 nd dose of CT treatment compared to other groups. A brilliant consistent relation between the elevation of IgG response and reduction of FEC was observed following the second booster dosing of the CT in G2. In conclusion, the CT evoked strongly in vitro and in vivo anthelmintic activity against H. contortus and could be used as a natural alternative treatment of high potency against haemonchosis in sheep.