Medhin Kiros, the presiding judge, said on Tuesday the court needed more time to assess the huge body of evidence in the case against Mengistu, who is accused of killing tens of thousands of people during a 17-year rule that ended in 1991.
"The trial has been adjourned until January 23, 2007, due to the overwhelming evidence presented ... the court needed more time," Medhin told the court.
Mengistu fled to Zimbabwe after being overthrown by a guerrilla army led by Meles Zenawi, the current prime minister. He has lived there in lavish seclusion since then.
Mengistu's rule began with the toppling of Emperor Haile Selassie in 1974 and included war, brutal purges and famine.
In the so-called Red Terror campaign in 1977-78, suspected opponents were rounded up, executed by garrotting or shooting, and their bodies thrown into the streets.