Handwriting expert Beate Wuellbeck told the Munich state court that only three letters in Demjanjuk's alleged identity card were clearly recognizable and she could not verify the authenticity of the signature.
Prosecutors say the signature on the identity card from the Nazis' Sobibor death camp is Demjanjuk's.
Demjanjuk, 90, is standing trial on 28,060 counts of accessory to murder for allegedly having been a guard at the Sobibor death camp.
He denies the charges, and the defense maintains the picture ID card, which was once in Russian hands, is a fake made by the KGB.
Wuellbeck told the court Wednesday she could not come to a definitive conclusion after comparing the recognizable letters in the signature with ten pages allegedly written by Demjanjuk.
Defense attorney Ulrich Busch requested another opinion from a specialist in Cyrillic script. Demjanjuk's native Ukrainian is written in Cyrillic. The court did not immediately decide on the motion.
Also on Wednesday, the defense team questioned the credibility of documents from Soviet intelligence services that placed a man named Demjanjuk had served at Sobibor between March 27, 1943 and September of the same year. That is roughly the same period in which Munich prosecutors say he worked as a guard at the death camp.
Defense attorney Ulrich Busch said a German court shouldn't consider documents from the totalitarian Soviet Union's agencies.
The prosecution argues that after the Ukrainian-born Demjanjuk, a Soviet Red Army soldier, was captured by the Germans in 1942, he agreed to serve under the SS as a guard. Demjanjuk claims he spent most of the rest of the war in Nazi camps for prisoners of war.
Presiding Judge Ralph Alt said Wednesday that the court plans to bring the trial, which started in November 2009, to a conclusion by the end of March.
A verdict could bring an end to a decades-long case, which started in 1981 when the U.S. revoked Demjanjuk's American citizenship, alleging he was a notorious Treblinka death camp guard "Ivan the Terrible" and had hidden the information when he immigrated to the United States.
He was extradited to Israel, where he was found guilty and sentenced to death in 1988, but the conviction was overturned five years later as a case of mistaken identity. He was later deported from the U.S. in May 2009 to face trial in Germany on charges that he was a guard at Sobibor, another death camp.
Juergen Baetz in Berlin contributed to this report.