Abstract: The growth and mortality rates of phytoplankton due to microzooplankton grazing and Noctiluca grazing were estimated at Guangxi (subtropical area) and Sanya (tropical area) in 2015. Results showed that the phytoplankton growth rates and mortality rates were high in both seas. The highest growth rates and mortality rates occurred along with bacteria. The ingestion rate of Noctiluca decreased gradually from bacteria, Synechococcus, picoeukaryotes and total phytoplankton. The grazing pressure of the microzooplankton on phytoplankton in the subtropical area is smaller than in the tropical area, indicating that the low temperature affects the feeding activity of the zooplankton. The nutrients are the major factor that causes the high growth rate.