The diplomat on Friday described it to Reuters as a "cookbook" for making the enriched uranium metal core of an atomic weapon.
"Also among the documents was one ... on the casting and machining of enriched, natural and depleted uranium into hemispherical forms," the report by IAEA Director General Mohamed ElBaradei to the IAEA board of governors, seen by Reuters, said.
The United States and European Union suspect Iran is developing nuclear weapons under cover of a civilian atomic energy programme.
Tehran denies wanting nuclear weapons, insisting its atomic ambitions are limited to the peaceful generation of electricity.
The Iranians told the IAEA they had received the document from individuals linked to the nuclear black market set up by Pakistani nuclear scientist Abdul Qadeer Khan.
Iran stated that the document had been provided on the initiative of nuclear black marketeers, not at its own request, the report said.
Iran says its nuclear project is
only for civilian purposes
Although this document shows how to make a vital part of an atomic weapon, there are many other parts it would need in order to produce an entire weapon, the diplomat added.
"Iran's full transparency is indispensable and overdue," the report said, repeating the wording used in the previous report on Iran issued on 2 September.
While Iran had been "more forthcoming" in providing access to documents and information in some areas, open questions on the peaceful nature of Iran's nuclear programme remain, the confidential report said.
The report said Iran should provide information and documentation on obtaining dual-use equipment and allow visits to various sites, including a site called Lavisan that the US says was used for sensitive nuclear work but which was bulldozed before IAEA inspectors could visit it.