The monograph is dedicated to one of the most important eras in the history of the Roman Empire and the entire ancient civilization. The profound transformations carried out by the eminent reforming emperor led to the beginning of a new period in Roman history. Principate - the state system that developed during the reign of Augustus, during the reign of Diocletian finally turns into the Dominant. The "monarchy in republican robes" becomes an unlimited monarchy. Based on a wide range of sources and studies, the author analyzes the evolution of imperial power, its new system, public administration, and the economic policy of Diocletian. Particular attention is paid to the religious policy of the founder of the Dominate, the last great persecution of Christians in the Roman Empire.