Louise Arbour said she had concerns about the fairness of the trial that convicted Hussein as well as Hussein's half-brother, Barzan Ibrahim al-Tikriti, and former chief Judge Awad al-Bandar.
Her appeal came amid reports that the two men could be hanged soon.
Iraqi officials have meanwhile said that at least one man has been questioned over the unofficial mobile-phone-camera video of Hussein's hanging on December 30.
National security adviser Muwaffaq al-Rubay'i was quoted as saying whoever released the video of the execution was trying to divide Iraq's Sunni Arabs and Shi'a.
The U.S. military, which handed over Hussein to Iraqis for the execution, on January 3 said it played no role in the hanging but would have handled the death sentence differently than the Iraqis.
In other news, two bombs exploded near a gas station in a mainly Sunni district of Baghdad today, killing at least four people and wounding more than 20.
Police said a car bomb went off shortly after the first blast, a roadside bomb that struck people lining up for fuel.
(compiled from agency reports)