A British woman died in the explosion in Baghdad.
A huge bomb exploded outside the building on Tuesday, killing 17 people, including the top United Nations man in Iraq, Sergio Vieira de Mello.
Despite this, the UN's secretary general said it would not be distracted by what he called a senseless and brutal act
More than 100 people were also injured in the blast, which destroyed a big part of the building.
A US military spokesman said they thought the explosion was a car or truck bomb - perhaps in a suicide attack.
Rescue workers have been working through the night because they think many people are still trapped.
The UN is an independent organisation, which means it isn't linked to any one country, but lots of people see it as a US body, which may be why it was attacked.