The present study examines the principle of impartiality in obtaining evidence in the Iranian criminal trial process. To achieve the principle of impartiality, we need to establish basic and preliminary conditions and principles so that, by implementing and respecting the principle of impartiality of judicial proceedings, the rights of citizens and the obligations of the right to a fair trial can be preserved and protected as much as possible. The research findings indicate that this principle, which has been recognized in the vast majority of domestic legal systems around the world and international legal regulations, especially specific regulations related to justice and fair trial, is one of the legislative dimensions of impartiality in criminal justice, which is rooted in the judicial law and is reflected in the status of judicial rights established for litigants in the rules of criminal procedure. Another dimension is the judicial dimension, which is a major part of the principle of impartiality in the acquisition of evidence, which is related to the trial stage and the practical process of criminal justice, and its manifestation, in addition to the behavior and speech of the judge, is the statements of the standard-bearers of criminal justice beyond the parties to the lawsuit, which usually have an impact on the judicial decision with their own opinions or statements. The results indicate that the manifestation of the principle of impartiality in obtaining evidence in the realization of public trial, the formulation of impartial laws; requires the impartiality of all judicial organs, including judges, lawyers, experts and other relevant members in the administration of justice. The effects of the principle of impartiality in obtaining evidence of the criminal legislator are, creating public trust in the judicial system, strengthening legal security and increasing the focus on justice in society, realizing an independent court and judge, guaranteeing the right to defense and the presence of a lawyer in the trial process, observing equality and guaranteeing weapons.