Bellemare takes on a tough challenge, with pro-Syrian opposition parties suspicious of the UN process and Lebanese politics in turmoil as parliament wrangles over electing a new president.
In a letter to the UN Security Council made public on Tuesday, Ban said he planned to appoint Bellemare, a former deputy attorney general of Canada, to succeed Belgium's Serge Brammertz as the next commissioner in the probe.
Hariri and 22 others died in a February 2005 Beirut car bomb blast that preliminary UN findings linked to Syrian and Lebanese security officials. Syria has denied involvement but the outcry forced it to withdraw its troops from Lebanon.
The UN team is also investigating other assassinations and bombings of the past three years that reflect Lebanon's protracted conflict between the anti-Syrian government coalition and the opposition led by the pro-Syrian Hezbollah group.
Anti-Syrian leaders led by Hariri's son, Saad, blame Damascus for the political killings. Syria's allies in Lebanon's opposition fear the Hariri case could be used for score-settling.
Bellemare's appointment comes amid a crisis over who will be Lebanon's next president. Parliament is due next week to elect a successor to pro-Syrian Emile Lahoud, but the country's divided factions have been unable to agree on a candidate.
The appointment also comes as preparations go ahead to set up a UN court to try suspects in the Hariri killing. The court approved by the Security Council five months ago is meant to start work by mid-2008.
The Netherlands has agreed to host the tribunal. The Hague is also the seat of the U.N. war crimes tribunal for the former Yugoslavia and the International Criminal Court.
In a separate letter today, Ban said he was nominating Brammertz to succeed Switzerland's Carla del Ponte as prosecutor of war crimes in the former Yugoslavia from the start of next year, a move that had been widely predicted.
REUTERS SBC VC2330