Abstract：Principle of Non-intervention is well accepted a basic principle of international law both in various international legal documents and legal doctrines, but is frequently violated nevertheless in international practice. Seeing from the traditional method of customary law which stresses the prominent role of actual practice of states, the contradiction between proclaimed principle and actual practice has posed special difficulties in identifying and proving the customary nature of non-intervention. Or more fundamentally, indeed which rule, intervention or non-intervention, is actually customary? Having examined five recent doctrines of customary law methods and relevant judgments of the International Court of Justice, the article submits that the principle of non-intervention is so fundamental to international society and of constitutional nature that justifies some special features in proving the customary law nature of non-intervention. The customary law nature of non-intervention derives from the sovereign equality of states being the structural principle of international society, and is further confirmed by the prevailing opinio juris. The element of opinio juris plays a dominant role other than state practice in developing and identifying the customary rules of non-intervention. However, state practice is highly relevant in determining the specific rules of prohibited interventions underlined by the principle of non-intervention. Moreover, the scope of state practice in the field of intervention should include not only the material practice of intervention and counter-intervention, but also practice of protest from the intervened and disapproval of international society at large. The article further asserts that the approach of customary law is value-oriented instead of value free in the contemporary inter-dependent international society.