The extensive tetracyclines in human and veterinary medicines cause contamination in the environment that could contribute to the antibiotic-resistant bacteria or the species competition in phytoplankton. In this study, Microcystis aeruginosa (bloom-forming cyanobacteria) and Chlamydomonas microsphaera (common green alga) were selected to test the effect of different concentrations of tetracyclines (tetracycline and oxytetracycline) in mono-culture and co-culture. The results showed that compared with mono-culture, the cell growth of C. microsphaera decreased signi cantly in co-culture treated with different concentrations of tetracycline and oxytetracycline. With the expose of 0.1, 2, 10 mg L -1 of tetracycline, the inhibition ratios of M. aeruginosa varied between 17.7% and 31.37% in co-culture compared to mono-culture, while the cell growth of M. aeruginosa was promoted treated with 0.1, 2, 7.25 mg L -1 of oxytetracycline in co-culture. However, the cell growth of C. microsphaera was signi cantly inhibited with all treatments in co-culture. With the treatment of tetracycline, the speci c growth rate of M. aeruginosa was 0.36 to 0.31 day -1 in mono-culture and co-culture, while that of C. microsphaera was ranged from 0.38 to 0.26 day -1 in mono-culture, and it decreased from 0.25 day -1 (0 mg L -1 ) to 0.08 day -1 (10 mg L -1 ) in co-culture. With the treatment of oxytetracycline, the speci c growth rate of M. aeruginosa was stimulated in co-culture while that of C. microsphaera showed an extremely signi cant inhibition in co-culture compared to mono-culture. Therefore, although M. aeruginosa outcompeted C. microsphaera in co-culture with the tetracyclines-free treatment, the competitive advantage of M. aeruginosa expanded upon addition of low or high concentrations of tetracyclines.