Withdrawal from the United Nations by member states is not provided for in the United Nations Charter. According to the Government Information Office of the Republic of China (Taiwan):
The U.N. Charter deliberately made no provision for the withdrawal of member governments, largely to prevent the threat of withdrawal from being used as a form of political blackmail, or to evade obligations under the Charter. Japan’s withdrawal from the League of Nations in March, 1933 (to signal its repudiation of the League’s condemnation of Japan’s invasion of China) was very much on the minds of the Charter’s drafters. (The other two major Axis powers, Germany and Italy, also withdrew from the League.) Some have questioned, therefore, whether it is even permissible for Members to withdraw from the U.N. The only other example of an effort to withdraw — by Indonesia in 1965 — actually tends to show that withdrawal, at least in the short term, has no force or effect.
The ROC holds this opinion against the fact of its removal from the UN to make way for the People’s Republic of China. See China and the United Nations.
Nevertheless, under customary international law, there exists the principle of rebus sic stantibus, or “things standing thus.” Under this principle, a state may withdraw from a treaty which has no withdrawal provisions only if there has been some substantial unforeseen change in circumstances, such as when the object of the treaty becomes moot or when a material breach is committed by a treaty party. Rebus sic stantibus has been narrowly construed (although not referred to by name) in Articles 61 and 62 of the Vienna Convention on the Law of Treaties. Therefore, under either customary international law or the Vienna Convention, it is unlikely that a state may unilaterally withdraw from the UN unless some fundamental change has occurred.