The explosion comes a month after a huge truck bomb at the building killed more than 20 people, including the UN’s highest representative in Iraq, Sergio Vieira de Mello.
The United States military said more than eight people had been wounded in the blast.
The blast happened around 250 metres from the UN headquarters, and destroyed at least six other vehicles.
US military spokesman Captain Sean Kirley said an Iraqi security guard had stopped the bomber before he reached the parking lot’s main checkpoint.
“The driver and the guard engaged in conversation and the bomb was detonated from inside the vehicle,” he said
“The damage to the other cars was catastrophic,” he added.
US forces were helping Iraqi personnel guard the headquarters at the time, but there were no American casualties.
The explosion came on the eve of the UN General Assembly in New York, which an Iraqi delegation was due to attend.
Three US soldiers were killed and a member of the Iraqi Governing Council was wounded in an assassination attempt over the weekend.
Iraqi Governing Council member Akila al-Hashemi, was critically injured when gunmen attacked her car in Baghdad on Saturday.
The on-going unrest in Iraq is putting pressure on President George Bush at home as he seeks a new UN resolution to create a multinational force for Iraq.
But France and Germany, opponents of the US-led war that toppled the former Iraqi leader Saddam Hussein, are demanding that America agree to a fast handover of power to Iraqis.
The United States believes a speedy power transfer would lead to failure.
Leaders of France and Germany stood firm on their demand at weekend talks in Berlin with Britain’s Tony Blair, casting doubt on whether talks with the United States this week will make progress.
Mr Bush, who is to address the UN General Assembly tomorrow, said he did not consider it essential to grant the United Nations a larger political role to obtain a new resolution backing the multinational occupation force.
Washington has said last month’s truck bombing was probably the work of Saddam Hussein loyalists or militants linked to Osama bin Laden’s al-Qaeda network.