To understand the effect of water temperature on growth pattern of Stephanodiscus sp., we weekly or biweekly investigated in the lower part of the Nakdong River from 1994 to 2006 and performed a laboratory experiment. Stephanodiscus was the most dominant species among phytoplankton in winter when low flow persisted and the high abundances of the species were maintained from December to February. Three strains of Stephanodiscus sp. were isolated for the in vitro experiment from the Nakdong River in January 2005. Over the water temperature range of $4^{circ}C$ to $20^{circ}C$, the growth patterns of Stephanodiscus sp. were different in the short-term batch culture. The maximum cell density of Stephanodiscus sp. was observed at approximately $5^{circ}C$ in the river systems, but the optimum water temperature of Stephanodiscus sp. was $10^{circ}C$ for the growth in the laboratory experiment. However, the proliferation of Stephanodiscus sp. was related to low water temperature in the Nakdong River.