Primary production of phrtoplankton was investigated in Naktong estuarine ecosystem monthly from October 1991 to September 1992 using carbon-14 uptake method. In spite of large spatial and temporal variation, there was a primary peak of net phytoplankton production in spring through early summer. The productivity was minimum of 543mg C·m^-2.day^-1 in September and maximum of 4112mg C·m^-2·day^-1 in April. The seasonal changes in factors regulating production in Naktong estuarine area were proposed based on the annual patterns of physical variables and nutrient concentrations. The late spring and early summer flowering of biomass and net production of phytoplankton was explained as the result of the balance between light and temperature. The high level of Production terminated in summer as the result of water clarity. Nutrient limitation may not be important, because of high nutrient level year around in this eutrophicated estuarine area.