"The IAEA experts started their visit to the Arak facility today morning," Fars news agency quoted Iran's atomic energy organisation spokesman Behrouz Kamalvandi as saying.
The visit comes just weeks after Iran clinched a landmark nuclear agreement with world powers under which it will freeze or curb some of its controversial nuclear activities in return for limited relief from crippling international sanctions.
Kamalvandi said the one-day inspection of the site 240 kilometres southwest of the capital would wrap up in the afternoon, and the International Atomic Energy Agency inspectors would leave for Vienna afterwards.
The IAEA regularly checks work on the Arak reactor, but says it has not received any new design details since 2006. And inspectors had not been to the heavy water plant at the facility since August 2011.
The Arak reactor is of concern to the international community because Tehran could theoretically extract weapons-grade plutonium from its spent fuel.