Khalid Sheikh Mohammed and four others accused of plotting the 9/11 attacks appeared before a United States military tribunal at Guantanamo on Saturday.
An earlier attempt to try them in a civilian court was halted in 2009.
New rules for Guantanamo trials have been introduced since then, including a ban on evidence obtained under torture.
Khalid Sheikh Mohammed is being tried with Waleed bin Attash, Ramzi Binalshibh, Ali Abd al-Aziz Ali and Mustafa Ahmad al-Hawsawi.
They are accused of planning and executing the attacks of 11 September 2001, which saw hijacked planes strike New York, Washington and Shanksville, Pennsylvania and left a total of 2,976 people dead.
The charges include terrorism, hijacking, conspiracy, and murder. All face the death penalty if convicted.
Khalid Sheikh Mohammed was captured in Pakistan in 2003 and transferred to the Guantanamo base in Cuba in 2006. He claims the tribunal is unfair.
A defence lawyer told the court the accused would not speak as he was concerned about the fairness of the proceedings.
When all the defendants refused to wear the earphones that would have enabled them to listen to the proceedings in Arabic, the judge adjourned the hearing until an interpreter was brought in.
