They were on trial along with dozens of other close associates of the deposed leader, accused of suppressing protests during the uprising.
Saif al-Islam was not present in court and gave evidence via video link.
He is being held by a former rebel group from the town of Zintan that refuses to release him.
Former head of intelligence for the Gaddafi regime, Abdallah al-Senousi, is among those also facing the death penalty, as is former PM Baghdadi al-Mahmoudi.
They will be given the right to appeal, according to the BBC’s John Simpson, who is in Tripoli.
Other defendants have received sentences ranging from five years to life imprisonment.
Source: BBC
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