Vice President and head of Iran's Atomic Energy Organization Ali Akbar Salehi and the Russian contractor for the power plant were on hand for the ceremony, the IRNA state news agency said.
Plant officials said the first of 163 fuel rods were installed in the heart of the reactor.
"Despite all efforts and policies of America and the European Union to put sanctions on Iran, the fuel of the Bushehr power plant will be loaded into its core," Iranian parliament member Alaeddin Boroujerdi told the news agency.
Iran insists it needs to enrich uranium, which also can be used to make weapons if refined further, to fuel future power stations designed to generate electricity and enable the country to export more of its gas and oil, Haaretz said.
Western powers say Iran's uranium enrichment is part of a covert weapons drive and have imposed increasingly tough sanctions on Tehran to force it to halt the work.