Understanding hydrological characteristics of climate change in Ethiopian highlands have a significance impact on water management practice. Due to climate change and variability, there was an increase in severity of extreme events which results in fluctuation of storage of water. This study is aimed to indicate the impact of climate change on extreme hydrological variables and it was carried out in Didessa watershed in Blue Nile basin. The study used the new emission scenarios based on Intergovernmental panel on climate change fifth assessment report with 50km resolution. The global circulation model output of representative concentration pathway 2.6 and 8.5 representative concentration path ways mission scenarios were used. The future projection period were divided in to three future time horizons of 2020`s, 2050`s and 2080 `s. The result indicated that the parameter efficient semi distributed physical model developed for the basin scale performs well with a value of coefficient of determination and (R2) = 0.65- 0.74 and Nash coefficient NSE = 0.75-0.78. The climate projection indicated that maximum and minimum temperature increases in all months and seasons in the selected watershed. The change in magnitude for RCP 8.5 emission was more than RCP 2.6 scenario. The study resulted considerable average monthly, seasonal and annual precipitation change variability in magnitude and direction. Based on this study stream flow is expected to decrease in the projection period. In 2020‟s, the average annual Stream flow projection decreases up to -48.12 % for RCP 2.6 and up to -11.52 % for RCP 8.5 scenarios. In 2050‟s also the average annual stream flow increase by 0.302 % and decrease by -1.57 % for RCP 2.6 and RCP 8.5 emission scenarios, respectively. Similarly, in 2080‟s, the average annual stream flow change decreases by -2.72% and -.11.52% for RCP2.6 and RCP8.5 respectively. For the future time horizon, the maximum Stream flow changes in wide range from (-48.48% to 1.72%) and minimum flow from (-0.34% to 1.14%) in both RCP2.6 and RCP8.5. Hence, the decrements of Stream flow volume in the watersheds would have a significant impact for the sustainability of existed and undergoing water development projects. Generally, a precaution of mitigation and adaptation measures ought to be developed for possible droughts in the area of the River basin.