The most important goal of the International Criminal Court is to realize criminal justice and deal with the impunity of criminals who have committed serious international crimes. In this direction, the International Criminal Court has assigned the task of preliminary investigation to the prosecutor. The current research tries to extract and analyze the principles and rules of the preliminary investigation system from the provisions of the Rome Statute, the policy document, the procedure of the prosecutor's office and other sources. The question is, what principles and rules govern the preliminary investigation system in the proceedings of the International Criminal Court? The present research has reached these findings by descriptive-analytical method and using library resources: Before starting the preliminary investigation, the prosecutor must verify a reasonable basis for starting the investigation in the preliminary assessment. Accordingly, the first rule is to verify reasonable basis criteria (admissibility, jurisdiction, and interests of justice) to start preliminary investigations. One of the general rules of preliminary investigation is that if a reasonable basis is verified, the prosecutor must obtain permission from the preliminary branch to start the investigation and conducted and completed the investigation under the supervision of the preliminary branch. In all these steps, the prosecutor and the prosecutor's office carry out investigations based on the principles of independence, impartiality, objectivity, transparency and with full knowledge.