The present research study was carried out for two years to investigate the physico-chemical parameters, density, and diversity of the plankton population and bacterial load of the Mirik Lake. Mirik, the famous hill resort in the Kurseung subdivision of Darjeeling District, West Bengal, India is very much popular for the "Mirik Lake" or "Sumendu Lake". This is the most beautiful lake in Darjeeling district where visitors come from different parts of the world. Besides attracting tourists, the lake water also serves as a major source of drinking water to the local people. This lake also helps in improving the economic status of the inhabitants of Mirik. Considering the length of the lake as well as point and non-point sources of pollution, seven sampling sites were identified for water sampling once every month at regular intervals. From the present investigation, it was confirmed that Sites 1, 2 and 6 were more polluted than the other areas. The results also suggest that the quality of the Lake water is not suitable for either drinking or bathing purposes. The long term scientific monitoring and public awareness are very much important to maintain the cleanliness and the health of the lake.