Abstract: Based on a survey on the species diversity, dominant species composition, cell abundance and horizontal distribution of Chaetoceros in the coastal waters of Hainan conductedduring May 28 and June 3 in 2011, we identified 28 species of Chaetoceros, among which 25 species were found in the northwestern waters, 22 in the eastern waters and 21 in the southern waters. The dominant species in these coastal waters were C.siamense, C.pseudocurvisetus, C.decipiens and C.lorenzianus, while C.anastomosans, C.rostratus, C.densus, C.affinis and C.messanensis were also abundant in these coastal waters of Hainan. Eastern waters had the highest cell abundance with an average number of (162.70415.16）104 indm-3, while northwestern waters was slightly lower in cell abundance with an average number of (102.81206.26）104 indm-3, and southern waters had a minimum value of（70.43142.09）104 indm-3. Horizontal distribution of Chaetoceros in these three waters had a declining trend from onshore to offshore. There was no significant correlation between Chaetoceros abundance and environmental factors.