Institutional delivery service utilization is critical in prevention of maternal death. However proportion of women utilizing Institutional delivery service in country in general and in southern nation in particular is very low. Reduction of maternal death is global priority particularly in developing countries including Ethiopia, where maternal mortality is one of the highest in the world. Low institutional delivery service utilization in pastoralist areas of Ethiopia caused by different factors. Objective To assess institutional delivery utilization and associated factors among mothers who gave birth in the last two years in pastoralist , Hammer woreda, south omo zone, Ethiopia. Methods: Community based cross sectional study with internal comparison were conducted . The study were conducted from December /2014 to January /2015.All women residing in the area at least for five months and who had delivered in the last two years were included in the study. Only 23.4% of the mothersgave birth to their last baby in the health facility. Residential set up[AOR=2.485, 95%CI: 1.325,4.659], maternal education[AOR=0.23795%CI: (0.09,0.622)], age at first pregnancy[AOR=1.8