Abstract: To explore the toxic effects of streptomycin, the growth and chlorophyll fluorescence were measured in Nostoc treated with different concentrations (0, 0.05, 0.10, 0.20, 0.50 and 1 mg/L) of streptomycin. The results indicated that the EC₅₀ of streptomycin to Nostoc (96h-EC₅₀) was (0.13±0.037) mg/L at the 96h. Streptomycin significantly decreased the content of chlorophyll a at all concentrations. In a concentration-dependent pattern, streptomycin significantly reduced the maximum photochemical efficiency (φP₀), variable fluorescence (F_v), potential activity of PSⅡ (F_v/F₀), active reaction center per unit area (RC/CS₀), the performance index based on absorbed light energy (PIabs) and the quantum energy yield of the light energy absorbed by the reaction center for electron transfer (φE₀). However, Streptomycin significantly increased the maximum fluorescence relative to chlorophyll a (F_m/Chl. a), the light energy absorbed by each active reaction center (ABS/RC) and heat dissipation (DI₀/RC) in a concentration-dependent pattern. A K-point at chlorophyll a fluorescence transient curve was found when the concentration is greater than 0.1 mg/L. These results suggest that streptomycin might affect the growth of Nostoc by inhibiting the electron transport of photosynthetic system Ⅱ (PSⅡ) and reducing active reaction centers, and that high concentrations of streptomycin showed an inhibitory effect on the growth of algae.