The man dubbed the "Butcher of Bosnia" will be asked to identify himself, answer questions about his health, and then plead to 11 counts of genocide, war crimes and crimes against humanity on the indictment.
For long Europe's most wanted man for atrocities committed during Bosnia's 1992-95 war that killed 100,000 people, 69-year-old Mladic was arrested in northeast Serbia last Thursday.
"Sixteen years is a long time to wait for justice," the prosecutor of the International Criminal Tribunal for the former Yugoslavia (ICTY), said after his dramatic capture.
|A Serbian police officer secures a convoy allegedly carrying Bosnian Serb ex-army chief Ratko Mladic from Belgrade to the city airport on May 31, 2011.|
Mladic is accused of masterminding the 1995 Srebrenica massacre of about 8,000 Muslim men and boys -- Europe's worst mass killing since World War II -- and the 44-month siege of the capital Sarajevo from May 1992 in which 10,000 died.
He was flown to the Netherlands on Tuesday to stand trial before the ICTY, after Serbian judges denied his appeal on health grounds and found him fit to stand trial.
On the eve of his much-anticipated appearance, however, Mladic's lawyer Milos Saljic said his client was treated for cancer two years ago while evading justice.
The ex-general had also suffered three strokes and two heart attacks, the lawyer said, as the prosecution of the tribunal warned the trial would not start for months.
"It is a complex case, it will take time to prepare," spokesman Frederick Swinnen said.
Mladic's one-time mentor, Serbian president Slobodan Milosevic, died in The Hague four years into his own genocide trial in 2006, of a heart attack.
If he fails to plead to the charges on Friday, Mladic will be given 30 days to do so or have the court enter a not-guilty plea on his behalf.
If he pleads guilty, there will be no trial, only a sentencing procedure. He faces a maximum penalty of life in prison.
It was not clear ahead of the hearing whether Mladic had appointed a lawyer or would opt instead to conduct his own defence, like Radovan Karadzic whose genocide trial opened in October 2009.
The two men, regarded as the political and military architects of the Bosnian Serb campaign, are charged with seeking to "permanently remove" Muslims and Croats from areas of Bosnia in pursuit of a "Greater Serbia".
As international news crews swarmed into The Hague, the tribunal rented extra space at a nearby conference centre to handle an overflow of visitors hoping to catch a glimpse of the ex-general in the dock.
Mladic's family are not expected among the throngs, having said through a lawyer they will visit him after the hearing, "calmly".
Five victims' representatives said they will be in The Hague to see Mladic in person.