This study aimed to detect the prevalence and susceptibility of gastrointestinal nematodes (GIT) against three major anthelmintic drugs. Therefore, a total of 1217 sheep from ten grazing flocks was examined for the detection of gastrointestinal parasites. A salt flotation technique and larval culture were carried out. Moreover, FAMACHA system was applied on the examined animals as a guide for anemia. Among examined sheep, a number of 80 infected animals were divided into four equal groups; control untreated group, albendazole-adminstered group-, levamisole-adminsetered group, and ivermectin- adminstered one. Meanwhile, a group of 20 animals, GIT-parasites free, were used as a control non-infected. The efficacy of anthelmintics was detected by fecal egg reduction test (FERT) 7 days post treatment. Findings showed that, the overall prevalence of gastrointestinal nematodes was 21.36%. Among them, Haemonchus contortus was present in 16.1%, Trichostrongylus spp < /em>. in 2.8%, Strongyloides papillosus in 1.7%, Cooperia spp < /em>. in 0.25%, Bonostomum spp. in 0.25%, and Chabertia ovina in 0.25%. There was a correlation between FAMACHA, PCV and fecal egg per gram. FERT was 100% by the 7^th day post treatment in the three treated groups. In conclusion, H. contortus was the highest GIT nematodes among the examined animals, and anemia induced by them could be detected with FAMACHA. Anemia of nutritional disorders and other non-parasitic causes have to be considered parallel with the application of FAMACHA on grazing animals. Based on findings of FER post treatment, animals were susceptible for three anthelmintics, albendazole, levamisole, and ivermectin.