Families of around 8,000 Muslim victims of the massacre were heading to the vast cemetery where more than 4,000 graves are scattered and 613 more had been newly dug to receive the remains of more victims during the commemoration service.
This year's anniversary of the 11 July 1995, massacre in Srebrenica, the worst mass killing in Europe since the end of the World War II, comes only weeks after the arrest of then Bosnian Serb army chief Mladic in Serbia.
Both Mladic and his political chief Radovan Karadzic, who was arrested in 2008, are charged by a UN war crimes tribunal with genocide, crimes against humanity and war crimes.
The Srebrenica massacre, the only episode in the Balkans wars to have been ruled a genocide by international courts, is a key part of their indictments.
The ceremony in Srebrenica will be attended by the Bosnian and Croat members of Bosnia's three man presidency, Bakir Izetbegovic and Zeljko Komsic, while hardline Bosnian Serb President Milorad Dodik will send only some representatives.