Owen, a former British foreign minister, was the European Community's peace envoy to the former Yugoslavia from 1992 to 1995 while the wars in Croatia and Bosnia were raging.
The former European envoy, who takes the stand on Monday, is the latest in a string of high profile political and diplomatic witnesses who have shed light on the chain of command in the Balkans.
Former Yugoslav president Zoran Lilic, top international envoy for Bosnia, Paddy Ashdown, along with William Walker, the US head of the international monitoring mission to Kosovo and Kosovo president Ibrahim Rugova have all appeared in
Milosevic has been on trial at the International Criminal Tribunal for the former Yugoslavia (ICTY) since February last year. He faces over 60 charges of war crimes and crimes against humanity for his role in the 1990s wars in Croatia, Bosnia and Kosovo that tore apart the Balkans.
For the bloody war in Bosnia that left over 200,000 people dead, he faces a separate charge of genocide.
Prosecutors have been trying to show that Milosevic controlled rebel Serbs in Bosnia and Croatia during the 1991-95 wars and thus can be held responsible for atrocities committed there.
In a written statement submitted to the court in September, Owen said he thought that Bosnian Serb leader and war crimes suspect Radovan Karadzic distanced himself from Milosevic in 1993.
Thousands of Bosnians were
widowed after Srebrenica killings
Before that time, Milosevic treated the Bosnian Serb leader "as someone largely under his control".
"After May 1993... I felt a change in Karadzic. He progressively became more independent of Milosevic," Owen said in the statement.
Owen wrote that in August 1993 it was "obvious that Milosevic's influence among Bosnian Serbs had waned... and (Bosnian Serb military leader Ratko) Mladic was increasingly difficult to control".
His testimony could bolster Milosevic's claims that he had no influence over the Bosnian Serb leadership at the time of the slaughter of thousands of Muslims in the Bosnian town of Srebrenica in July 1995.
Owen's peace role
During the Balkan wars his peace plan drawn up with ex-US secretary of state Cyrus Vance flopped after being rejected because it appeased Serbian aggression.
Owen was also crticised for his failure to say whether genocide was committed against the Bosnian Muslims or whether they were massacred in Srebrenica. The former peace broker believes that the matter should be left to an international tribunal.
Unlike the other witnesses, Owen was not called to give his evidence by the prosecution but by the trial chamber itself. In a statement he said he wanted to be called by the court instead of appearing as a witness for the prosecution so as to preserve the impartial position of international negotiators.
Owen, 65, is now a member of the House of Lords, the British upper chamber of parliament. His testimony is expected to take at least two days.