The water hyacinth challenge is widespread throughout many tropical and subtropical areas of the planet. It is an invasive, floating aquatic plant that poses a substantial threat of unanticipated water loss and highly impacts on physicochemical water quality of a standing water body. The study aimed to evaluate the evapotranspiration rate of water hyacinth and water quality deterioration on Lake Tana (the largest fresh lake in Ethiopia). The experiment was carried out in two artificial ponds (i.e., control and with water hyacinth), for each pond had a 1-meter width, 1-meter length, and 0.94 meters in height. The pond water level measurement was carried out for three months. The sample for water quality analysis was taken from the surface of Lack Tana by using judgmental sampling methods. Samples were taken at four points: with water hyacinth and without water hyacinth. The statistical analyses of dissolved oxygen with water hyacinth were 6.65 ± 0.44 mg/l; without water, hyacinth was 7.93 ± 0.42 mg/l with a p-value of (p < 0.0058). The total dissolved solid with water hyacinth was 90.6 ±1.06mg/l, and without water, hyacinth was 92.1 ± 0.54mg/l, with a p-value of (p<0.05). The temperature of the Lake with water hyacinth was 23.70 ± 0.42 0 c, and without water hyacinth was 22.08 ± 0.33 0 c with a p-value of (p<0.0057) and the PH of the Lake with water hyacinth was 5.53±0.53, and without water hyacinth was 6.53±0.40 with a p-value of (p<0.0021). The experiment showed that the water loss by water hyacinth was 1.6 times greater than free water surface evaporation from the control pond. The result of water loss by water hyacinth, in a 1 m 2 area of the pond was 0.28 m 3 / month. Then, the estimated water loss by water hyacinth on Lake Tana was minimally about 779,240 m 3 /month and maximally 7,012,600 m 3 /month. However, water loss by water hyacinth on the Lake was comparatively very low, because the water loss by water hyacinth on Lake Tana for evapotranspiration is more suitable than from the experiment station. Generally, Water hyacinth is exceedingly difficult to eliminate, and most management efforts should aim to eradicate it from the water bodies of the Tana basin. Keywords: - Water hyacinth, Evapotranspiration, physical water quality, Lake Tana