The combination of cross-section and controlled field trail were carried out from December 2015 to June 2016 at Haramaya District, Eastern Hararhge, Ethiopia to evaluate the prevalence ofgastrointestinal nematodes and anthelmintic efficacy test in naturally infected small ruminants using faecal egg count reduction (FECR) test. From a total of 372 faecal samples examined using fecal flotation technique about 57%(212/372) samples were positive to gastrointestinal nematodes. Coprological investigation revealed that sheep and goats in district were infested with varieties of nematode parasites. The strongyles species were highly prevalent nematode parasites identified at about 22.3%(83/372) followed by trichuris 9.1%(34/372) and strongyloides 5.6%(21/372); whereas mixed infections of strongyles and strongyloides were recovered at lowest rate which accounted for about 5.1%(19/372). From considered risk factors age and kebele showed significant association (p< 0.05) with prevalence of nematodes parasites. The study revealed that 53.29%(97/182) of goats and 60.53%(115/190) sheep were found positive, while 58%(115/198) in female and 55.74%(97/174) in male and showed no significant difference (p> 0.05) between these factors. Mc Master egg counting in sheep harbored nematode parasites showed relatively similar percentage with no significant difference; but more of examined animals were classified in the range of moderate to heavy infections. The faecal egg count reduction test against albendazole, Tetraclozash and ivermectin indicated that 97.59%, 98.79% and 99.56% respectively which indicated an overall of very good state of efficacy against gastrointestinal nematodes of sheep. Therefore this study revealed that small ruminants of the study area harbored considerable level of gastrointestinal nematodes parasites which need further studies on species identification, larval ecology and strategic deworming with proper practice of available anthelmintics in small ruminants of the study area.