This article examined the interactive effect among herbs extracts in combined use, compared with the effect of individual herbal extracts on 1,1-diphenyl picryl hydrazyl (DPPH). The Chinese traditional prescription, four substances decoction (FSD) consisted of 25 g of Angelica sinensis (Umbelliferae, AS), 6 g of Ligusticum chuanxiong Hort. (Umbelliferae, LCH), 10 g of Paeonia loactiflora Pall. (Paeoniaceae, PCP), and 15 g of Radix rehmanniae preparata (Scrophulariaceae, RRP). Each herb was quantitatively mixed according to the FSD prescription ratio. The interaction between the herbal extract in two-two combination way were estimated using a spectrophotometer. The results showed the strongest synergistic effect in LCH and PLP system, with IC 50 0.25 mg/ml, and the additive effect was obvious in the AS and LCH combined system. The other combined system showed additive effect or sub-additive effect. This implied that the combined system effect have a close relationship with the single component activity.