The increasing growth of environmental pollution and destruction and the formation of successive environmental crises caused the governments to seriously think about a solution almost four decades ago, based on this international environmental law as one of the branches of international law. Public was formed by the international community through the establishment and implementation of binding and non-binding legal rules, and in the last few decades, it has been developed in terms of content, form and structure, considering that the importance and fundamental role of the environment in human life and development is undeniable. Therefore, the basis of the global decision to protect the environment became concrete with the first United Nations World Conference on Man and the Environment named "Stockholm Convention" in Sweden in 1972, so that the human right to enjoy a healthy environment is equal to Human rights were recognized. Based on this, in the current research, we intend to answer this basic question by using the analytical descriptive method with the help of library tools, what is the position of legal rules and especially customs in international environmental law? The research hypothesis states that customs had an important impact on the formation of rules and agreements resulting from international environmental law.