Iran had been watching the developments in Lebanon since Lebanese President Emile Lahoud signed an amnesty on Tuesday pardoning Geagea.
Tehran claims Geagea and his Christian Lebanese Forces militia kidnapped the diplomats and delivered them to Israel.
Along with Geagea, nearly three dozen Muslim fighters, some with alleged links to al-Qaida, were pardoned, although it is not clear when they will be released from prison. Geagea had been in jail since 1994.
“Filing a lawsuit against perpetrators of kidnapping of four Iranian diplomats is on the agenda,” said Hamid Reza Asefi, an Iranian Foreign Ministry spokesman said on Thursday.
“We are reviewing claims against Samir Geagea in this regard.”
The four diplomats, Mohsen Mousavi, Ahmad Motevaselian, Taqi Rastegarmoghaddam and Kazem Akhavan, were kidnapped in Beirut in 1982.
Geagea led the powerful militia during the 1975-90 civil war. He has spent most of the past 11 years in solitary confinement in an underground cell at the Lebanese Defence Ministry.
He was arrested in April 1994 and the Lebanese Forces outlawed after a bombing at a church killed 10 people. He was acquitted of that bombing but convicted on other charges.
He is serving three life sentences for the assassination of political rivals in the civil war, including the bombing of a helicopter that killed then prime minister Rashid Karami in 1987.