Objectives : This experiment was designed to investigate the effect of Gamisoyo-san(Jiaweixiaoyaosan, SYS) on serotonin activity of P815 Mast Cell. Methods : The effects of SYS on activation of TPH-1 mRNA and AAADC mRNA in P815 mast cell were investigated. The effect of SYS on content of serotonin in P815 mast cell was investigated. The effects of SYS on activation of DPPH and SOD in P815 mast cell were investigated. Results : 1. The SYS increased the activation of SOD and DPPH in P815 mast cell. 2. The SYS decreased the manifestation TPH-1 mRNA in P815 mast cell. 3. The SYS decreased the manifestation AAADC mRNA in P815 mast cell. Conclusions : This experiment shows that Gamisoyo-san might not be effective for treating depression in terms of biogenic amine theory. However, Gamisoyo-san showed significant anti-oxidative effect and it can not yet be ruled out for treating depression. Therefore, pathogenesis of depression and clinical research of Gamisoyo-san is suggested for future research.