Data on the water quality at reef corals in cDastal area of Phu Yen province archieved in December 2008 (rainy season) and April 2009 (dry season) showed that DO concentrations were relatively high (6 mg/l), concentrations of nitrate (from 24 to 48 ugN/I), phosphate (from 1.9 to 15.3 ugP/l) and coliform density (from 0-4600 MPN/100ml) comparatively varied widely, ammonia and nitrite were not found, heavy metal concentrations (Zn 15 ug/I, Cu and Pb 2
0 ug/l) were fairly low, hydrocarbon concentration was comparatively high. In generally, there were no differences in values of basic parameters (DO, pH, TSS), nutrients, heavy metals (Zn, Cu and Pb) and hydrocarbon between two seasons while Fe and Si concentrations were higher and BOD5 was lower in December 2008. Mouth of Xuan Dai Bay was the most affected by fresh water from continent. According to criteria for coral reef conservation, water quality was relatively good although concentration of nitrate (both seasons) and phosphate (dry season) and coliform density (rainy season) were slightly higher than correspondent critical values.