In all countries the need of legal experts is accepted. Countries have passed specific laws in this field. This research, with analytical-descriptive method, seeks to find the answer of how the judicial experts system is regulated in the Roman and Common law legal systems? Comparative studies shows that the median line of the judicial experts system in the world is the traditions accepted in the two great Roman and Common Law. In this research, we are trying to analyze the judicial experts in these two legal system and then analyzing them. In the Roman law the expert is considered as the judge's assistant, and all aspects of judicial experts are regulated in the most extensive form by the state. The existence of a specific list of experts and the role of judges in referring to experts can be considered the most important components of the expert system in these countries. In contrast, in the common law system, according to the tradition of the sitting judge, regulation is done in a minimal way. Respecting the comparative findings, the judicial expert system in Iran is facing a duality. Theoroticaly like Common law, but the relevant laws indicate the acceptance of Roman traditions.