An engineer at the Baiji Refinery was killed in the assault, The New York Times said.
The disruption of production at the refinery, which produces 150,000 barrels of oil derivatives a day, could fuel anti-government protests in the country, the BBC said. One major grievance has been erratic fuel supplies, and the oil industry is also a major source of government revenue.
The attackers, equipped with handguns and silencers, overpowered night-shift employees at 4:30 a.m. and planted eight bombs, the Times said. The bombs started a fire.
The refinery is in a Sunni area in Salahuddin province, 100 miles north of Baghdad. Until recently, officials said much of its production was diverted into the black market, helping to fund al-Qaida in Iraq.
A pipeline bombing this month cut production at the Dora refinery in Baghdad. The country's third major refinery is in Basra in southern Iraq.