Extracts of 11 medicinal plants were used to test the inhibitive effects on the growth of Chlorella pyrenoidosa. The study results show that the extracts of Coptis chinensis, Paris quadrifolia, Dryopteris setosa, and Stephania tetrandra had inhibitive effects on the growth of Chlorella pyrenoidosa. When the relative concentration of the extracts was at 1g/L, the range of the median lethal times LT50 was in the following order: Stephania tetrandra < Paris quadrifolia < Coptis chinensis < Dryopteris setosa. In the experiment, the algal response to the relative concentration gradient was tested, all the algae died after 3 days when the relative concentration of Stephania tetrandra was at the level of 2g/L. The study results also indicate that the effects of inhibition were not changed markedly by the storage time and storage temperature. Stephania tetrandra showed the greatest inhibition on the growth of the algal species among the medical plants studied. Thus, Stephania tetrandra has the great potential in inhibiting algal outbreak. The other three medical plants tested in the study also showed moderate inhibitive impact on the algal growth, indicating that they are also valuable in control and management of algal populations.