The changes in water quality parameters for ecosystems have a substantial impact on the existence and survival of species. Water quality is influenced by geological, hydrological, climatic and anthropogenic factors. The rapid industrialization, growing urbanization and increasing use of chemicals in agriculture, constitutes some of the important factors responsible for various forms of pollution of water bodies. The aim of the study was to evaluate Chlorophyll-a concentration and other related Water Quality Parameters of Lake Tana, Ethiopia. The water samples were taken from specific tributary rivers and places 10 km distance away from the inlet of the river mouths. The samples were collected four times from August to April 2018/19, using standard sampling methods and preservation technique. The samples were analyzed based on method number 150.1 of Environmental Science Section. The method involved in the assessment of Chlorophyll-a, Turbidity, Secchi disc depth, DPC, Nitrate and Dissolved Oxygen water quality parameters of Lake Tana were in-situ measurement, laboratory measurement, statistical analysis and mapping by using ArcGIS. The laboratory analyses were conducted using Palintest Photometer, turbid- meter, and Spectro-Photo meter. Results reveled all parameters have higher concentration during the wet season than in the dry season and also have spatial variability throughout the lake ecosystem. Chl-a concentration, Turbidity, Dissolved Phosphorus concentration and Nitrate water quality parameters have their mean values above the permissible limit and Secchi disc depth, dissolved oxygen water quality parameters below the permissible limits of EPA and WHO standards in both dry and wet seasons. The negative relationship between Chl-a concentration and dissolved oxygen were observed and the relationship between Secchi depth, turbidity and nutrients show that significant positive relationship in both seasons. The trophic state of Lake Tana indicates pollution. The lake‟s trophic state is in between mesotrophic and hypereutrophic state. Generally, Lake Tana water quality is decreasing from time to time. Therefore, the concerned authorities should establish a mechanism to protection, prevention and monitoring of Lake Tana water quality.