In accordance with Articles 130 and 131 of the first edition of the Constitution of the Islamic Republic of Iran (adopted in 1979), the first vice president - who is now Mohammad Mokhber - steps in and assume the duties of the president if the president is unable to fulfill his legal duties due to dismissal, resignation, absence, illness, or death.
The Constitution stipulates that these responsibilities are transferred to the First Vice President with the approval of the Leader of the Islamic Revolution, and in 50 days the country needs to go to an election to elect a new president.
According to the Iranian political hierarchy, the head of the state is the Leader of the Islamic Revolution, Ayatollah Seyed Ali Khamenei, and the president is considered the head of the government, the second-in-command.
In case of sudden death, the first vice president is expected to guide the country through this transitional period until new presidential elections are held.
The Iranian constitution was amended in 1989, when important changes were instituted.
Source: Tehran Times