The era of about 40 years of Mongol’s military campaigns in Iran and its neighboring territories has been one of the most eventful periods of history and thus has attracted serious attention from historians. In addition to describing these military campaigns, historians have tried to record the invasions' chronology and times. However, their reports on this case, i.e. the time of the happening of the events related to the military campaigns, has suffered from at-times considerable discrepancies. Based on this hypothesis, the present descriptive-analytical research, tries to shed some light on the chronology of Mongol conquests of Persia, by comparing the reports of historical sources written in Iran, Iraq, and Mamluki territories (Egypt and Levant) to answer the question of the differences and similarities observable in the chronology of these campaigns. The results show that during the period of the Genghis Khan's invasions (616-621 AH), Iranian historians played a more prominent role than Iraqi and Mamluki historians but in post-Genghis era up to the Hulagu Khan's campaign (624-651 AH), the role of Iraqi historians was more outstanding and ultimately during the Hulagu Khan's campaign (651-658 AH), Mamluki historians played an important role in writing a chronology of the Mongol conquests.