Voting began on Sunday morning in election that will see the pro-European Democratic Party of President Boris Tadic go up against conservative Progressive Party headed by Tomislav Nikolic.
Surveys just ahead of the election show Tadic, leading with 58 percent, while Nikolic had 42 percent.
Incumbent Tadic is seeking his third and final five-year mandate which he said would be dedicated to his country's European integration and economic development.
He came just ahead of Nikolic in the May 6 first round of presidential elections, in which the nationalist has accused his rival of "vote fraud".
This comes while the state electoral commission has rejected allegations that the ruling party has defrauded 500,000 votes.
The outcome of the vote may affect Serbia’s EU prospects and it could also determine whether Serbia will continue to reconcile with its neighbors and wartime foes, including Kosovo.
The European Union wants Serbia to loosen its grip on a northern slice of Kosovo, which declared independence in 2008.
Tadic announced his resignation in April, in an attempt to pave the way for an early presidential vote alongside the other polls.
Economy is the most important issue for some 6.7 million people eligible to vote in Serbia.