At the beginning of the humanity, the changing natural balance of the world has also changed the climate. Global climate change, which threatens all life and its environment, is likely to have a significant impact on our water resources. It is therefore important to examine the precipitation and streamflow from the elements of the hydrological cycle as well as the changes in temperature in time. In this study, Büyük Menderes Basin, which is the largest basin of the Aegean Region with about 26.000 km2 area, was examined. In long-term average monthly precipitation, temperature and stream flow data belong to Aydın, Denizli and Uşak provinces which are taken up in the basin, were analyzed by using Mann Kendall Trend Analysis. Accordingly, while the most significant increase trend in temperature was in Denizli with seven months, the increase trend in summer months in Uşak was statistically significant, too. An increased trend was found in temperatures for Aydın, which was not statistically significant. When it comes to evaluate precipitation and streamflow trends; while increase trend was not found significant in Denizli and Uşak, significant decrease trend in stream flow was found in two stream gauging station in the basin.