The freshwater blooms mainly blue-green algal blooms occur frequently in the lower Naktong River in summer, which provoke many socio-economical problems; therefore, the early detection of bloom events are demanding through the quantitative and qualitative analyses of blue green algal species. The in vivo fluorescence properties of cultured strains of Microcystis aeruginosa, M. viridis, M. wesenbergii, M. ichthyoblabe, Anabaena cylindrica, A. flos-aquae, and Synedra sp. were investigated. Wild phytoplankton communities of the lower Naktong River were also monitored at four stations in terms of their standing stocks, biomass and fluorescence properties compared with its absorption spectram. The 77K fluorescence emission spectra of each cultured strains normalized at 620 nm was very specific and enabled to detect of blue green algal biomass qualitatively and quantitatively. The relative chlorophyll a concentration determined by chlorophyll fluorescence analysis method showed significant relationship with chlorophyll a concentration determined by solvent extraction method ($R^2$ = 0.906), and the blue-green algal cell number determined by microscopic observation ($R^2$ = 0.588), which gives insight into applications to early detection of blue green algal bloom.