The objective of present study was to determine prevalence and intensity of gastrointestinal nematodes in naturally infected sheep and associated risk factors in and around Hirna, western Hararghe, Ethiopia. 384 faecal samples were examined using egg flotation technique. Out of which 272 (70.83%) were found positive for gastrointestinal nematode infection. Coprological investigation revealed that sampled sheep were infested by a variety of helminth nematodes. Strongyle type were the most frequently (58.59%) recovered nematode eggs followed by Strongyloides (23.44%) and Trichuris species (2.08%). The eggs per gram (EPG) count was determined using McMaster technique which showed that 133 (48.90%) of the sheep were lightly infested, 96 (35.29%) sheep were moderately infested and 43 (15.81%) sheep were heavily infested. There was significant association between the gastrointestinal nematode infection and animals with different body condition scores. Animals with poor body condition had a significantly higher worm egg count (p < 0.05) as compared to sheep in moderate or good body condition. There was no significant difference (p > 0.05) in prevalence between different sex and ages. The present study showed that ovine gastrointestinal nematodes are of the major cause of helminthiasis in the study area. To get clear epidemiological picture detailed study should be conducted to clearly identify parasitic nematodes using faecal culture and postmortem examination in the study area as well as farmers should be educated on the importance of the parasitic diseases, its economic losses and the correct ways to improve animal husbandry system.