Abstract: The theory of the monarch, minister, assistant, and guide is the core content of formula theory, and it is of great significance for the study of compatibility law of formulas. By conducting research on the literature related to the theory of the monarch, minister, assistant, and guide, the compatibility and effects of monarch medicines in representative formulas were analyzed. It is believed that the theory of the monarch, minister, assistant, and guide applied in formulas in The Yellow Emperor's Classic of Internal Medicine is not the same as that of herbal medicine mentioned in Shen Nong's Classic of the Materia Medica. In addition, the law of priority, subordination as well as correlation in the theory of the monarch, minister, assistant, and guide applied in formulas is analogous to the rule of ancient forms of government, but not equal to the concept of state in feudal society. The monarch medicine of a formula is determined under the premise that "the medicine in a formula having the leading action in treatment is called monarch", logically followed by the complement that "medicine with potent action in the treatment is called monarch".