Background: Stress reduces physical and mental tolerances (immune potential) of humans and it induces progression of existing illness or causes latent disorders to become active. Thus, the control and suppression of stress plays an important role in the improvement of quality of life and prevention of diseases. Ginseng, oriental bezoar and glycyrrhiza have been used for Kampo (herbal treatment) for thousand years and a number of pharmacological and clinical studies have reported their effects. However, it has not been previously described how the combination of these most commonly used herbs affect mental stress. Objective: This is a randomized, double-blind, placebo-controlled experiment to examine the effectiveness of reducing stress response by taking Kampo. Ten healthy males (mean age 27±1) participated in the study. The effectiveness of stress reduction was assessed by measuring ECG, salivary chromogranin A (CgA), blood glucose, WBC, granulocytes, lymphocytes, NK cell activity, etc. Salivary and blood measurement values of pre- and post-mental arithmetic stress were compared. In addition, ECG measurement values of pre- and mid-mental arithmetic stress were compared. Results: we observed a higher HF power and a lower SNS index, HR, CgA, WBC and granulocytes in the Kampo trial than those in the placebo trial. The HR, HF power and SNS index were changed significantly (p<0.05) and CgA, WBC and granulocytes tended to show some differences between the two trials (p<0.1). However, blood glucose, lymphocytes, and NK cell activity showed no significant differences between the Kampo and placebo trials. Conclusions: The result suggests that the Kampo should be useful in reducing mental stress.