The aim of this paper is to have a better understanding of the significant trends of stream flow in the Upper Blue Nile Basin (UBNB) for 40 years using statistical tools. Analyzing the stream flow is important as it gives a wide vision of a catchment’s response and helps in suggesting suitable future water resources management. Data from two stream flow stations in the basin were analyzed using the nonparametric Mann-Kendall (MK) statistical trend test to determine positive or negative trends in the stream flow data series. The MK statistical test showed that for most of the year there is no significant trend for the stream flow time series data except for two months in the rainy season for the selected stations: Bahir_Dar station and Kessie Station. By applying Sen’s estimator, it was found that a decreasing in the trend slope occurred in Bahir_Dar station while it increased for Kessie Station.