The institution of interdiction (hajr), owing to its extensive effects on legal capacity and the fundamental rights of persons, is regarded as one of the most sensitive institutions of civil law and non-contentious matters. The Iranian legislature has subjected the adjudication of petitions for interdiction to special formalities, extensive investigations, and heightened supervision. The judgment of the Third Branch of the Supreme Court of Iran in a case of interdiction on grounds of insanity is a clear example of proper judicial supervision and safeguarding of the fundamental principles of judicial proceedings. In this judgment, the Supreme Court, emphasizing the mandatory role of the public prosecutor, the supervening nature of insanity, the necessity of up-to-date investigations, and observance of defense rights, identified fundamental deficiencies in the judgments of the lower courts, which led to quashing of the judgment under cassation appeal. This article, adopting an analytical-descriptive approach and drawing on the Constitution, the Civil Code, the Non-Contentious Matters Act, and the Code of Civil Procedure, examines the judgment of the Third Branch of the Supreme Court of Iran and, while explaining the theoretical foundations of fair trial, demonstrates its implications for judicial practice of courts in adjudicating petitions for interdiction.