The investigating judge, as one of the key components in the preliminary investigation phase of criminal proceedings, plays an essential role in ensuring a fair trial. In both the legal systems of Iran and France, this institution faces differences in its structure, powers, and limitations. This research, using a descriptive-analytical method and based on library resources, aims to conduct a comparative analysis of the capacities and challenges of the investigating judge in the two legal systems of Iran and France, and to assess its impact on the principles of fair trial. In Iran's legal system, despite some reforms in the Code of Criminal Procedure enacted in 2013, challenges such as the dependency of the investigating judge on the prosecutor, the non-adversarial nature of investigations, restrictions on the parties' access to the case file, and the lack of effective protective mechanisms for witnesses and informants are evident. In contrast, the French legal system, with structural reforms such as the full independence of the investigating judge, ensuring active participation of both the defendant and the complainant in the investigation process, providing the right to appeal the decisions of the investigating judge, and allowing multiple investigating judges in complex cases, has been more successful in achieving the principles of a fair trial.