The Lake Tana basin is located in the north western part of Ethiopia. It comprises an approximate area of 15,114 km2 including Lake Tana. The basin is a high tourist destination site particularly to see old monasteries hosted by many islands and the Blue Nile Fall. Lake Tana is also known by its diverse fish species. In addition to its ecological values the lake is used for water supply, fishing, irrigation, hydro power and many other ecosystem services. However the basin is facing many threats. One of these threats is change in land use and land cover. The specific objectives of this study are to assess land use and land cover change in 1986, 2001 and 2015, and to quantify the impact of this change on the runoff in the basin. ERDAS IMAGINE and ARC GIS used for land use and land cover change analysis, and SWAT model simulation for quantifying the impact of land use land cover change on the runoff. The major land use and land cover types identified in the basin are farm land and settlement, shrub land, range land, forest, bare land, wood land, wet land, and water body. The results of the study showed that farm land is continuously increased from 34.8 % in 1986 to 44.3 % and 55.1 % in 2001 and 2015 respectively. Conversely wood land decreased persistently within these years. Forest cover, wet land and bare land increased between 1986 and 2001 but decreased between 2001 and 2015. Shrub land and range land decreased initially between 1986 and 2001 but increased between 2001 and 2015. Water body slightly decreased between 1986 and 2001, and has the same coverage between 2001 and 2015. Consequently the mean annual and wet months (JuneV September) runoff decreased in the 2001 LULC from the 1986 LULC by 0.2 mm and 0.6 mm respectively. This is mainly due to the increase of forest cover although farm land and settlement cover also increased in the 2001 LULC from the1986 LULC. But the mean annual and dry months (October-May) base flow decreased in the 2001 LULC from the 1986 LULC by 0.8 mm and 0.7 mm respectively. Then mean annual and wet months (June-September) runoff increased in the 2015 LULC from the 2001 LULC by 2.1 mm and 5.8 mm respectively. This is primarily due to the decrease of forest cover, and the increase of farm land and settlement cover in 2015 LULC from the 2001 LULC. Accordingly the mean annual and dry months (October-May) base flow decreased in the 2015 LULC from the 2001 LULC by 3.9 mm and 2.3 mm respectively. The other major threat on the water resources of the basin is climate change. The change of land use and land cover, and climate, and their impact on the runoff and base flow in the basin has many implications on Lake Tana. For example, if the runoff amount increased, consequently the sedimentation to the lake will also be increased. Sedimentation leads to many negative impacts to the lake’s water and fish resources. Climate change will decreased on lake precipitation and surface water inflow, and increased evaporation, then the lake’s water balance will be disturbed and resulted in to water shortage. In the opposite on lake precipitation and surface water inflow will be increased although evaporation also increased, then it resulted in to lake level rise and flooding which especially affects the downstream countries.