Abstract: In this paper, an experimental research is reported on the effect of water extract of Cladonia alpestris and S-(-)usnic acid on Trichomonas vaginalis in vitro. The results showed that both the water extract of Cladonia alpestris and S-(-)usnic acid exhibited a strong effect against Trichomonas vaginalis in vitro .As the time of action of the agents was prolonged , the mortality of Trichomonas vaginalis increased.For S-(-)usnic acid, 0.4 mg/ml was the lowest effective concentration against Trichomonas vaginalis in vitro .No remarkable differences were found in the effect of the S-(-)usnic acid and metronidazole at concentrations of 0.4 mg/ml and 0.6 mg/ml.