Criminal psychology is a branch of psychology that focuses on the psychological, personality, and behavioral factors that affect the commission of a crime, analyzing the individual roots of delinquency. By examining the psychological states of criminals, deviant behavioral patterns, and the mental structure of criminals, this science plays an important role in a deeper understanding of the nature of crime and the motivations for committing it. In criminal law, criminal psychology can be used as a tool to identify, prevent, and even treat criminal tendencies. Iran and France each benefit from this knowledge in some way, but the extent, structure, and manner of its use differ from each other. This research seeks to answer the question of what effect criminal psychology has on the commission of a crime and how do the Iranian and French legal systems use its capacities in the criminal process? The aim of the present study is to comparatively examine the extent and manner of using criminal psychology in the analysis, prevention, and investigation of crimes in these two countries. The research method is descriptive-analytical and its information was collected using library resources, scientific articles and comparative studies. The findings show that France, by utilizing specialized institutions, criminal psychologists and specific legal procedures, has provided a more effective position for this science in the criminal process; while in Iran, the application of criminal psychology is limited and often summarized in the form of a forensic psychiatry degree. As a result to improve the level of criminal justice and effectively prevent crime, it is necessary for the Iranian penal system to provide the basis for institutionalizing the application of criminal psychology by modeling itself on more advanced structures such as France.