SOURCE: BBC News (5-10-10)
It said the information had come from Eulex, the EU police mission in Kosovo, and Serbia was sending investigators.
The victims are believed to have been killed during the 1998-99 conflict, when Serbian forces fought ethnic Albanian rebels in Kosovo.
The grave is near the town of Raska, close to the border with Kosovo.
There had been rumours two years ago that the grave existed, but searches at that time found nothing.
The war crimes prosecutor's office said it would be several days before exhumations could begin at the site in Rudnica, about 180km (110 miles) south of Serbia's capital, Belgrade.
Officials said the remains were buried beneath a building whose foundations had been deliberately constructed to hide the site, reports the BBC's Mark Lowen in Belgrade.
The prosecutor, Vladimir Vukcevic, said the discovery was a sign that Serbia was committed to coming to terms with its history.
"This is more proof that Serbia does not shy away from its dark past and is ready to bring to justice all those who have committed crimes," Mr Vukcevic was quoted as saying by AFP news agency.
Our correspondent says identifying the victims through DNA analysis is likely to take several more years - and prolong the painful period of reconciliation.
It is not the first time mass graves from the conflict have been found in Serbia. The bodies of more than 800 Kosovo Albanians were found in several locations in Serbia in 2001, including police compounds.
The bodies were moved out of Kosovo before a Nato bombing campaign forced Serbian security forces out of the region.
Other, smaller mass graves have been found containing Serbian victims of ethnic Albanians.
Researchers in Serbia and Kosovo say more than 11,000 people died in the Kosovo conflict, most of them ethnic Albanian, but at least 2,300 Serbian.
A further 1,800 people are classified as missing, according to Eulex figures, but are presumed to be dead.
A former top Serbian police official, Vlastimir Djordjevic, is currently on trial at the UN's Hague-based war crimes tribunal for the former Yugoslovia.
He was allegedly involved in the murders of hundreds of ethnic Albanians and the deportation of 800,000 others from Kosovo during the conflict, when he was in charge of police forces in Serbia.
He denies charges of deportation, murder and persecution.
He was a close aide to the late Yugoslav President Slobodan Milosevic, who died in tribunal custody in 2006 before a verdict was reached in his trial for war crimes.
Belgrade withdrew forces from the Serbian province of Kosovo in 1999 after a Nato bombing campaign, and the area was put under UN control.
Kosovo's declaration of independence in 2008 has been recognised by more than 50 countries, including the US and most EU states, but not recognised by more than 100, including Serbia and Russia.
Recent Serbian governments have been pro-Western and last year the country submitted a formal application to join the EU.
But membership negotiations cannot begin in earnest until two war crimes suspects - including the former Bosnian Serb military commander Ratko Mladic - have been captured.