The author’s study f inds the issue of whether, in the Thai legal system, judges of the Court of Justice should apply the proportionality principle in adjudication in constitutional cases as in European countries. Regarding the results of the research, the comparative analysis of the laws and precedent of the courts between Germany, France, England, Ireland and Thailand indicates that the legal system of Thailand accepts the constitutional right model of Principles and Rules proposed by Professor Robert Alexy in his Principle theory of constitutional rights, as observed in the other legal systems due to the fundamental cause that Thailand is a democratic state as others. Moreover, from th e precedent of the Supreme Court of Thailand denying the supremacy of parliament explicitly and inspecting other state exercising powers affecting rights of people intensively, the Thai legal system utmost takes after the German legal system with certain f laws, i.e., inconsistency and opacity when applying the Principle of Proportionality in the judgement.