The visit is the second to Iran in three weeks and is seen as a key test of Iran's willingness to hold substantive talks with the International Atomic Energy Agency about suspicious activities outlined by the agency in a November report.
The team is headed by chief UN nuclear inspector Herman Nackaerts, who before boarding the plane for Tehran in Vienna expressed hope that "concrete results" would emerge from the trip, but cautioned that progress "may take a while."
"We hope to have a couple of good and constructive days in Tehran," Nackaerts told reporters at Vienna airport.
"Importantly we hope for some concrete results from this trip. The highest priority remains of course the possible military dimensions of Iran's nuclear programme, but we want to tackle all outstanding issues," he said.
"This is of course a very complex issue that may take a while. But we hope it can be constructive."
Western diplomats to the IAEA said that the first trip, from January 29-31, was more of a confidence-building exercise, with the inspectors neither meeting key figures in Iran's nuclear programme nor visiting any nuclear sites.
The IAEA's November report said Iran had carried out activities that were "relevant to producing" a nuclear weapon.
Since its publication, the United States and the European Union have ramped up sanctions on Iran's oil sector, and speculation has grown that Iran's arch rival Israel might launch military attacks on Iranian atomic facilities.
The Islamic republic, which denies seeking the bomb and which says the IAEA report was baseless, last week defiantly trumpeted advances -- downplayed by Washington as "hyped" -- in its nuclear programme.
Iran also signalled on Sunday that it is ready to hit back hard at sanctions threatening its economy, by announcing it has halted its limited oil sales to France and Britain.
Iran said last week it was ready to resume stalled talks on its nuclear drive with the P5+1 powers -- the US, Russia, China, France, Britain and Germany -- which broke down in Turkey in January 2011.