Tana basin has been identified as one of the growth Corridor of the country by the federal democratic republic of Ethiopia in 2005 and accordingly several hydropower and irrigation projects have been developed and implemented. Consequently, the lake level and outflow change from time to time and environmental problems have been increasing and some development sectors such as wetland, navigation, and tourism continued to be affected due to the dynamic nature of the lake. In response to this, a study to investigating the lake level and outflow hydrological variability and changes on Lake Tana has been conducted using long time series records the data are lake level data (1973 to 2020) and outflow data (1977 to 2014) collected from Abbay basin then dividing the data into twotime categories (before and after1996-construction of the Chara-Chara weir).To analysis this study SPSS, Minitab, and spread sheet are used for inferential statistics and descriptive statistics of the study. The results showed that lake level before 1996 was stationary (Pvalue=0.03) while after regulation it showed non-stationary (P-value=0.11). Similarly, the statistical analysis result for the outflow indicated stationary results before regulation and non-stationary after regulation. Moreover, the highest mean lake level difference was found in the month of November (0.498 m) followed by December (0.489 m). Likewise, the average monthly outflow variability has risen dramatically in drier months than the wetter months. Such variability might have been caused by the regulation activities done at Chara Chara weir. Nevertheless, the mean difference for outflow was higher before regulation compared with after regulation for the months from August to December and the maximum mean difference in these months was found in April 52.9 m 3 /s. while the mean difference of outflow before regulation from January to July was found lower compared to after regulation and the maximum mean difference in these months was found in October (42.33 m 3 /s). In general after regulation outflow from Lake Tana has increased by about 94 percent compared with the outflow before regulation. The differences between these changes could be attributed to a variety of factors, including climate change, the need for water for agricultural use, pollution, and a new invasion of water hyacinth. As a result of the dynamic fluctuations in lake level throughout time, Lake Tana must be carefully managed and used for various purposes. Keywords: Chara- Chara, lake level, outflow, before regulation, after regulation