This article studied the changes of chemical components in decoctions that made from the single medicinal herb of Rhizoma Alismatis or Evodia rutaecarpa, the co-decoctions made from different compatibilities of the two medicinal herbs, and that mixed decotions of two single medicinal herb decotions, by HPLC. The changes of chemical components of the codicotions were focused on. The relationships between the changes of chemical components of the codicotions and defferent compatibility ratios of medicinal herb were summarized.