Water pollution is the main problem that effects on the community in Thailand. The Pollution Control Department in Thailand has indexed the water quality using eight parameters. In this paper, the main propose is to reduce eight parameters of water quality index(WQI) to four parameters which are dissolved oxygen (DO), total solid (TS), biological oxygen demand (BOD), and suspended solid (SS). The factor analysis, correlation analysis and fourier series are used to simulate the data. The data obtained from Tha Chin river during 2002 - 2007 is used in this model. The genetic algorithm is applied to find the weight of each parameter. The result shows that the modified WQI using four parameters provides the same result obtained from the model using eight parameters.
