The new Unita leader is expected to contest elections against President Jose Eduardo Dos Santos, which could be held next year.
General Paulo Lukamba Gato became Unita leader after the death of Jonas Savimbi last February, followed shortly afterwards by that of his deputy, Antonio Dembo.
This led to an end to the country's 27-year war, which left some one million people dead.
Correspondents say that General Gato's withdrawal from the leadership race leaves two front-runners: Isaias Samakuva, Unita's former representative in Paris, and Abel Chivukuvuku, a current member of parliament.
Lively debate
The leader will be elected in Unita's first ever peace-time congress, to be held in May or June.
"I became Unita's leader during a crisis and my leadership will end with the election of a new president," General Gato said at the end of the movement's political commission.
The BBC's Justin Pearce in Luanda says that the lively debate at the meeting showed a break from the authoritarian days of Jonas Savimbi, whose portrait was in the meeting-room.
Some 110,000 former Unita fighters have left the bush and are in demobilisation camps along with their families, where some people have starved to death because of food shortages.
"The government spends absolutely nothing on them (the former combatants). They (the government) want to have peace but want it to come cheaply," General Gato said.