The main subject of this article is an investigation of the origin of the co-regency institution in the Seleucid kingdom. Taking as a source some works of ancient writers (Appian and Plutarch), Babylonian documents (chronicles and astronomical diaries), epigraphical and numismatical data, special importance of this institution during the early period of Seleucid kingdom formation could be evidently observed. The processes in social, economical, ideological sphere, and the condition of domestic and foreign policy in the Seleucus state (from the end of IV BCE till the beginning of III BCE) could be regarded as the reasons for the origination of co-regency.