Since the principle of common but differentiated responsibility emerged into the domain of international environmental law, it has been adopted by several international environment instruments. However, contentions about its nature in international environmental law remain. Most Chinese scholars consider it as a binding principle in international environmental law, but many foreign scholars doubt it. The content of this principle, specially the standards of and reasons, is ambiguous. Its applications are inconsistent and rather limited. Therefore it is necessary to distinguish those practices adopting the CBDR principle from those that did not refer to the CBDR principle clearly. Based on the theory of the sources of international law, it is believed that the CBDR principle has not yet evolved into a customary international law. Nevertheless, it is still significant in international law.