The objectives of this study carried out in 2011 were to study the variation of water quality along the distance of the river and seasons from the upper part down to the lower part of the river before flowing into the Gulf of Thailand. The water sample collection and analysis were accomplished conforming with the American Public Health Association (APHA) methods and the indicators of water quality for analysis were DO, BOD, NH₃-N, TCB and FCB. The surface water quality standard of Thailand was used for the assessment of water quality. The study results indicated that most of the water in the Chao Phraya river was of lower quality than the established water quality standard. There were only 22% of water samples the quality of which reached officially established standard. The hypothesis testing found that the distance along the route of water mass had relationship with water quality and seasonal variation had no statistically significant relationship to water quality at 0.05.