In order to understand variation and relationship between standing crops and biomass of phytoplankton dominant species for the long term periods, this study was seasonally investigated in the marine ranching ground of Tongyeong coastal waters from 2000 to 2007. Total 268 taxa representing 217 Bacillariophyceae, 46 Dinophyceae, 4 Dictychophyceae, 1 Euglenophyceae were observed in phytoplankton communities. Dominant species consisted of 5 species as standard in standing crops: Chaetoceros curvisetus (18.01%), Chaetoceros socialis (12.95%), Skeletonema costatum (8.39%), Chaetoceros compressus (6.87%), Asterionellopsis glacialis (5.02%). However, to determine dominant species as biomass concept, Ditylum brightwellii, Guinardia striata, Rhizosolenia spp. and Skeletonema costatum were occupied with dominant species (19.67%). As determining for cell sizes, dominant species were divided with two groups such as micro- and nanophytoplankton (standing crops) and mesophytoplankton (biomass). However, Skeletonema costatum in anophytoplankton was associated to affect fluctuation between standing crops and biomass.