The news comes the day after International Atomic Energy Agencyinspectors discovered more traces of highly enriched uranium in Iran, a development that could strengthen US claims that Iran is secretly trying to develop atomic weapons.
IAEA spokeswoman Melissa Fleming said: "The Iranian government requested the delay to allow them more time to prepare for the visit."
She did not provide more details.
The IAEA had on September 12 given Iran until October 31 to answer all its questions on a nuclear program suspected of hiding an attempt to develop atomic weapons.
Among these questions was a request for Iran to account for traces of highly enriched uranium that could be weapons-grade found at a facility making uranium fuel.
Iran said Monday that it had launched a trial run at this facility, in Natanz, 250 kilometres (150 miles) south of Tehran, despite the IAEA request for it to cease enrichment activities.
The mission of a senior IAEA delegation that was to leave Vienna Sunday was to be the first wave of over a month of intense inspections right up until the October 31 deadline.
But in a surprise development, it was revealed Thursday that results from IAEA tests already conducted in Iran had found more traces of highly-enriched uranium.
It was not certain whether this uranium had been produced by Iran or had come from contamination of imported equipment, diplomats said.
At stake is whether Iran is producing highly enriched uranium for nuclear weapons, as the United States claims, or whether the traces came along with second-hand equipment bought abroad, as Tehran says.
The traces were found by IAEA inspectors in the first half of August at the Kalaye Electric Company near Tehran, said the diplomats who asked not to be identified.
The Iranians claim this site is not involved in their nuclear program but was used for storing equipment, they added.
The United States has charged that the Iranians have used Kalaye to test centrifuges used to make highly enriched uranium that can be used to make atomic bombs.
"This news (of traces at Kalaye) is big news perhaps but it didn't lead us any further into having answers," a diplomat close to the IAEA said.
The Iranians claim that their nuclear program is peaceful.