This research attempted to analyze the variation of surface water quality in Kelani River and finally to create a mathematical model for the Water Quality Index (WQI) by using the data that have been obtained. Kelani River is the second largest river in Sri Lanka while being the most polluted river in the country. It is one of the main sources of drinking water for human beings while being the home for many aquatic species. The ecological aspects of Kelani River are threatened due to the increasing number of industries along both sides of the watershed. The present study is conducted to analyze the water quality in Kelani River using six parameters including pH, Turbidity, Temperature, Total phosphate, Nitrates, Dissolved Oxygen (DO), and ultimately finding the Water Quality Index (WQI) by collecting 5 sets of samples from seven locations from March 2017 to August 2017. The highest WQI (72.90) was reported near the Water Treatment Plant in Biyagama, while the lowest WQI (62.98) was achieved near the Water Board Pump in Ambathale. Water quality of five of the samples were in good quality range and the other two samples were in medium quality range. A mathematical model for WQI was created using the above mentioned six water quality parameters.