The main contenders on Sunday are government candidate Dragoljub Micunovic, a veteran of Serbia's democracy movement, and a candidate 25 years younger from the nationalist Radical Party, Tomislav Nikolic.
Neither candidate commands any significant support, according to opinion polls - so the contest may be whether enough people vote to make the election valid.
Serbian law requires at least 50% of the Balkan country's 6.5 million registered voters to cast their ballots.
Two presidential elections have already failed in the past 13 months because of chronic voter apathy.
Surveys indicate about 60% of the electorate are undecided about their political allegiances.
Analysts are divided over whether the election will succeed or not, although one poll last week showed that it might just squeak over the turnout threshold and save the country from yet another humiliating re-run.
"I hope that Serbia today is determined to do the job and choose a new president. If the turnout is sufficient I will win in the first round," Micunovic told reporters as he cast his ballot in downtown Belgrade.
European diplomats said the real interest would be in what clues it offered for the outcome of parliamentary polls on 28 December.
Serbs sceptical that elections
will improve the country's situation
"The first stage of democratic transition [from communism] comes to an end with these elections," said one, adding he doubted Sunday's vote would succeed.
The reformist government of Prime Minister Zoran Zivkovic has been fighting for its life since the assassination of former premier Zoran Djindjic earlier this year.
The murder shattered any illusion of unity with the 15-party alliance which has ruled Serbia since former strongman Slobodan Milosevic, ousted from power in 2000.
Faced with a no-confidence motion in parliament and without a majority in the 250-seat assembly, the government last week chose to call early elections a year before its term is due to expire.
Serbian leaders admit that reforms designed to bring Serbia, a republic of 10 million people, into NATO and the European Union will be on ice until a new parliament is in place.