Ahmadinejad has repeatedly promised to announce "good nuclear news" in the near future, and is widely expected to use his visit to the heavily defended uranium enrichment plant to do so.
Any major announcement of progress is likely to further heighten tensions with the West, which fears Iran is seeking nuclear weapons and wants Tehran to suspend enrichment.
But the Islamic republic insists its nuclear drive is solely aimed at supplying energy for a growing population.
Iranian officials have remained tight-lipped over the nature of the news, which is expected to relate to progress towards enriching uranium on an industrial scale at Natanz.
The UN Security Council has already imposed two packages of sanctions against Iran over its failure to heed ultimatums from the world body to suspend uranium enrichment.
Uranium enrichment is highly sensitive because the process can be used both to make fuel for nuclear energy plants and the explosive core of a nuclear bomb. Iran insists it has every right to the full nuclear fuel cycle.
Last April Iran announced it had succeeded in enriching uranium to 3.5 per cent, good enough for nuclear fuel but still well off the 90 per cent levels required to make an atomic weapon.