The refinery's senior engineer, Hussam Ali Jalmod, says the refinery opened Friday and will start producing in the next few days.
He says the plant's initial capacity is 10,000 barrels per day. It plans to double that by mid-2009.
Iraq has the world's third-largest oil reserves, totaling 115 billion barrels. But it suffers acute refinery shortages following years of U.N. sanctions and war.
The country currently refines just over 400,000 barrels per day at three main refineries and handful of smaller ones.
Qadisiyah province is 80 miles, or 130 kilometers, south of Baghdad.