Object To investigate the isomer transition reaction of ferulic acid and the existing form of ferulic acid in Ligusticum chuanxiong Hort. and Angelica sinensis (Oliv.) Diels. Methods HPLC and HPLC-MS were used to confirm the isomer transition of ferulic acid. Results The trans-ferulic acid was translated into cis-ferulic acid partly when its solution was deposited under some routine conditions,and the trans- and cis-ferulic acids could coexist in the Chinese medicinal materials,L. chuanxiong and A. sinensis. Conclusion The trans-ferulic acid is stable thermodynamicly,but could be translated into cis-ferulic acid partly and slowly.