At least 19 people were killed when a car bomb went off near a bus carrying Iraqi security forces.
A spokesman from the US military said the explosion happened near Balad, a town to the north of Baghdad.
The attack took place in an area called the Sunni Triangle, which has seen lots of attacks in recent days, including one which killed at least 29 people.
Rebel fighters have been very active in the run-up to Iraqi elections, which are planned for 30 January.
It's thought they are against the elections because they are backed by the US, who they want to pull out of Iraq.
Iraqi police and National Guards will be trying to keep the elections safe.