This study aims to apply economic evaluation of water quality by Contingent Valuation Method in the waterways in Bangkok by using contingent valuation method survey. The water quality is the local public goods, thus the survey had carried out in Bangkok where the waterway locating. The pre-tested questionnaire was distributed directly to respondents and the data collection have been conducted in the last quarter of the year of 2004. The survey collected information on the individual information and environmental evaluation of the respondents. This study examines the determinants for WTP for water quality improvement. The result shows that individual income and location of resident have positive influence on WTP. Further, individual income change rate appears to be positively associated with WTP change rate. Although their individual income has been increased in the year of 2004, higher than the year of 1999, the respondents are not willing to pay more for public good. The trend of mean of Willingness to Pay changed from 199 Baht in the year of 1997 to 110, 88 108 and 126 Baht in the year of 1998, 1999, 2003 and 2004 respectively. From this changing curve we could attempt to calculate the total WTP of environmental quality in the period of time. More importantly, we could find the value gain experienced by the average WTP when a water quality improvement is achieved. The total benefit value of a specific improvement could then be estimated by multiplying this average WTP value by the number of population. This benefit can then be compared against the cost of achieving such a quality improvement to see if it was worthwhile. Finally, we notice that the willingness to pay of non-impact sample is not zero. This is due to the fact that such respondents, while not personally wishing to avoid the impact, nevertheless do value its continued existence and even upgrading so that others can enjoy its benefits. This non-use, “existence” value derives from people’s altruistic “public preferences” showing that the concentration upon people’s “private preferences” as demonstrated by the market prices of marketed goods does not always fully capture the entire range of values which people have for things