Great White Pelican (Pelecanus onocrotalus) is a resident species in Ethiopia. However, the available information on the breeding ecology and its status is inadequate. This study was carried out in Lake Tana area and Koga Water Reservoir from 2020-2021. The study aims to investigate the breeding biology, density and distribution of P.onocrotalus. A total of 35 transects was carried out to estimate the abundance and density of P.onocrotalus. MCDS model was employed for distance analysis. Different statistical tools were used to evaluate the different parameters. ANOVA, GLM, MANOVA and t-test were applied. The result showed that all P.onocrotalus nests were located only in inaccessible islands far from human disturbance. The active nesting time was throughout the year with peak breeding season of late December to the end of March and June to the end of July, in both dry and wet seasons. The inter distance between nests did not vary between breeding islands. The nest morphology parameters were not statistically different. The nesting density was 3.64/100m2. The average clutch size of P.onocrotalus was 2.14 (N=98). The mean length, width and weight of eggs (N=98) were 90.33±.44 mm, 58.70±.24 mm and 173.68±1.86 g, respectively. There was strong positive correlation between egg length and egg weight and statistically highly significant (R=0.71, P<0.001, N=98). Hatchability was 85.71%, but the pre-fledged percentage was about 37.76%. There was highly significant difference in the hatching success among clutch sizes (F=7.97, df=3, P<.001). There was spatial and seasonal variation in the distribution of P.onocrotalus and the variation (77.04%) in the distribution brought by the predictor variables study sites, seasons and habitats. There was a significant difference in population size between dry and wet seasons (t = 2.904, df = 222, P<.001). The population was more abundant during the dry season. The mean distribution of Great White Pelicans in the study sites were statistically highly significant (F=11.83, df=5, P<.0001). During the dry season, the pelican density was estimated in the lake 0.553±0.151 ha-1, wetland 0.064±0.065ha-1, floodplain 0.176±0.073 ha-1, water reservoir 0.010±0.006 ha-1, and the total population was estimated to be 2766.0±754.09,159.00±161.25, 882.00±362.90 and 25.00±15.978, respectively. In wet season estimation of pelican density in lake, wetland, floodplain, and water reservoir habitat were 0.510±0.159; 0.048±0.050; 0.151±0.062 and 0.014±0.009 ha-1 respectively, while the total population (N) was estimated to be 2550.0±793.63, 120.00±125.5, 753.00±309.16 and 35.00±22.46, respectively. The total density (D), abundance (N) and estimate of density of cluster (DS) of Great White Pelican in both dry and wet seasons were estimated. The pooled estimates of the density of P.onocrotalus was 0.255 ha-1 and 0.231 ha-1 with estimated population 3832 and 3458 pelicans in dry and wet seasons, respectively. However, the pooled estimate of the density of clusters of pelicans (DS) in dry and wet seasons was 0.099 and 0.131, respectively. Fikir Mefija Islands are identified as the main breeding and feeding sites, while Shesher and Welala floodplains and Bahir Dar lakeshore are the main feeding sites. However, the breeding islands are disturbed by fishermen and farmers shrink the feeding floodplain habitats. Over and illegal fishing, agricultural encroachment and population growth are the main threats.