The visit by the International Atomic Energy Agency team to the plant, which is being built near the Shiite holy city of Qom, was announced on Wednesday by Iran's envoy to the IAEA, Ali Asghar Soltanieh.
The inspection is the second by the IAEA in less than a month. Four inspectors first visited the plant on October 25 after its disclosure by Iran to the agency triggered intense outrage in the West.
"This site will from now on be under the IAEA. And for your information there will be tomorrow another inspection of this site in order to make sure that we are fully cooperating," Soltanieh told reporters in Vienna on Wednesday.
The Fordo plant, named after a nearby village where large numbers of Iranians were killed during the war with Iraq in the 80s, is guarded by anti-aircraft guns and is being built inside a mountain.
Iranian officials say the construction of the plant is a message to the West that Tehran will never give up its uranium enrichment work and that the plant is a back-up facility in case the main enrichment plant at Natanz is bombed.
Washington and arch-foe Israel have never ruled out a military strike against Iran's nuclear facilities which they suspect are being used to make weapons, a charge strongly denied by the Islamic republic.
Soltanieh has said that Iran has no other enrichment plants apart from Fordo and Natanz.
The expected UN inspection comes a day after Iran rejected plans for it to send most of its stocks of low-enriched uranium abroad under the IAEA-brokered deal.
Foreign Minister Manouchehr Mottaki said on Wednesday that Iran has ruled out proposals backed by the major powers for it to ship out more than 70 percent of its stocks before receiving any nuclear fuel in return, the ISNA news agency reported.
Mottaki said Iran is prepared to consider the idea of a simultaneous exchange of uranium for fuel but the UN nuclear watchdog, which has been brokering the negotiations, has already said that idea is unacceptable to the Western powers.
The Western governments support the UN-brokered deal because they believe it would leave Iran with not enough stocks of low-enriched uranium to be able to make a bomb.
