Abstract: Nalanda Lake is (Long. 80º 37’ 1.93” and Lat. 7º 40’ 30.6”) one of the ancient Lake in the Sri Lanka. It’s fed by the Basin of Mahaweli River. This lake water is use for drinking, agriculture and human activities. Generally, in this lake observing the high amount of algae due to high nutrient content. Equally lake water had a less amount of Dissolved Oxygen (DO). The DO concentration is characterize of a lake healthiness. And also, the DO concentration responds to the Biological Oxygen Demand (BOD) load. Main Objective of this research was determined the DO concentration in selected Non-point source and developed the DO consumption model by using Mathematical Equation. Five different sample location points were selected. DO and BOD variation was studied during four weeks. Totally 20 samples were analyzed. DO was measured by DO meter (HACH Sension6), BOD was measured by the Winkler method. DO consumption was measured using Streeter-Phelps equation. According to investigated data DO vary (8.31±0.76 – 10.02±0.46 mg/l). Minimum DO value (7.62 mg/l) obtained point-3. BOD varied from 5.13±1.08 to 5.92±0.53. The position where the lowest DO appeared was around 14-18 km.