Field study on a comparison of population ecology of Beisa oryx (Oryx beisa) was conductedrneveiy month from October to March, 2005 for six months in Ulala Sala and adjacent areas inrnAwash National Park and for five months (November to March, 2005) in the selected study siternin Alledeghi Wildlife Reserve. The result indicated that an average of 220 individual of Beisarnoryx in Awash National Park and 335 individuals of Beisa oryx in the selected study site inrnAlledeghi Wildlife Reserve. The mean group sizes of 17.6 individuals were found in the selectedrnstudy site in AWR, and 13.9 individuals were at Illala Sala and adjacent areas in Awash NationalrnPark. The difference in mean group sizes of Beisa oryx between the two study areas was notrnsignificant. However, they were found to be significant for mixed herds (P=0.008). The grouprnstructure of the animals varied with seasons in Awash National Park and the selected study site inrnAlledeghi Wildlife Reserve. Large herds were frequently observed during short rainy season inrngrassland and smaller groups during the dry season in both study sites. Variation in monthlyrngroup size did not exist in both study sites, The most cohesive herd is that of the female and herrnimmediate young. Single males were most common at Illala Sala and its adjacent areas in AwashrnNational Park and male-female group in the selected study site in Alledeghi Wildlife Reserve.rnThe mean number of calves in Awash National Park and the selected study site in AlledeghirnWildlife Reserve showed significant differences between the two seasons at 0.05 level ofrnsignificance (P=0.039 and P~0.014). The females during dry and wet seasons are more numerousrnthan the males in both study sites. The animals were observed utilizing the grassland, shrubrngrassland and wooded grassland habitats in both seasons in both study sites, however, there wasrnmore utilization of grassland habitat (72.8% in ANP and 89.1% in Alledeghi Wildlife Reserve).rnDistribution of group sizes in grassland and shrub grassland habitats of the two study sitesrnshowed a high significant differences (P=0.003 and P=0.025, respectively). Grassland habitat wasrnthe best preferred habitat of Beisa oryx. The changes in the distribution of habitat preference inrnthe two seasons showed that no significant differences. However, there was slightly significantrndifferences in the group sizes in grassland habitat of ANP between the two seasons (P=0.047).rnSeasonal home range size varied in ANP. Wet season home range size (31km ) was larger thanrnthat of dty season (25km ), Minimum Convex Polygon (MCP) method was used to calculate the home range. On the average about 52.61 % of Beisa oryx daytime were spent in feeding andrn36.65%) in rest i .e. standing and lying down.