Guibi-tang is a traditional medicine used for the treatment of colds. We investigated the levels of several compounds in Guibi-tang before and after fermentation with Lactobacillus and tested their ability to inhibit angiotensin-converting enzyme. Six known compounds (decursin, decursinol angelate, nodakenin, liquiritin, formononetin, and 6-gingerol) and 2 unidentified compounds were detected in Guibi-tang (GB) and fermented Guibi-tang (FGB) by an established HPLC-DAD method. The levels of the 6 known compounds were decreased after fermentation. FGB showed more potent inhibition of angiotensin-converting enzyme activity than GB. In conclusion, fermentation with Lactobacillus affects the content of several compounds in GB and improves its angiotensin-converting enzyme inhibitory activity.