Water quality and biological characteristics of phytoplankton in the central coastal region has been studied based on the five years monitoring data (from 2006 to 2010). For water quality, the main pollutants are COD and oil and grease which had concentrations exceeded the standard values. Phytoplankton taxonomy analysis reveals that there were 144 species belonging to 19 families, 4 orders, 3 classes and 3 phylums. Biological indicators (H', D) reflect that the phytoplankton biodiversity in study area was relatively low, stable population, there is no dominant species. Pearson correlation coefficients show positive correlation between the phytoplankton density and numbers of water quality parameters such as Redfield index, nitrogen and silicate nutrients and a negative correlation with other parameters: Temperature, salinity, T55, pH and oil content. This result is valuable for predicting phytoplankton primary production depending on the change of environmental quality. Except for pH which could affect to the phytoplankton biodiversity and dominant, other water quality parameters on the other hand did not appear to have any particular influence on phytoplankton production. This conclusion requyres a more comprehensive study of the environmental quality, light condition, marine hydro-dynamic, zooplankton... to the extent of phytoplankton biodiversity of the study area.