Demjanjuk, 89, was deported in May from the United States to Germany, where he was wanted for alleged involvement in about 29,000 killings as an SS guard at Sobibor, a World War II Nazi death camp in Poland.
Munich State Prosecutor Anton Winkler said that although deemed fit for trial, doctors have restricted the time he can be tried each day to two sessions of 90 minutes each, CNN reported.
Demjanjuk, a native Ukrainian, has long denied any role in the Holocaust and claimed he was a prisoner of war, not a death camp guard.
This would be Demjanjuk's second war crimes trial. He was convicted in an Israeli trial in the 1970s and sentenced to death but the sentence was overturned when mistaken identity was proven.