We observed a severe reduction in salinity around Khang Khao Island at the end of the rainy season in September and October, 1995, caused by the most serious freshwater flooding in Thailand in recent 15 years. During low tide, corals around Khang Khao Island were exposed to 23‰ at three meters depth, and 25‰ at five meters. As a result, Pocillopora damicornis and a few species of Acropora showed colony bleaching or partial or complete colony death in the field. On the other hand, Porites spp., the dominant corals of this area, did not show any conspicuous effects. Porites lutea could survive five days of experimental exposure to 17‰, the critical salinity for other corals. The chlorophyll-a concentration of P. lutea that become partially bleached after seven days exposure to 20‰ was higher than in normal control fragments. These phenomena suggest that P. lutea has a compensation mechanism against salinity reduction. Both field observations and experiments suggest that lowered salinity caused by runoff during the rainy season is an important factor besides turbidity in establishing the characteristics of the coral community structure around Khang Khao Island. A detailed study of the process of coral community formation in this area may contribute to an understanding of not only the negative effects of continental great rivers on the establishment of coral reefs around tropical continental shores but also the influence of the heavy rainfall on a coral community in the coral reef region by the global climate change.