The adjournment after some 40 minutes, was the second postponement since March 24, and came as a rights group voiced doubts over the possibility of a free trial.
As was the case in March, the postponement was due to a number of defendants, notably Seif, being absent and is meant to give lawyers time to prepare their cases.
In all, only 23 of 37 charged were in the dock, sitting behind bars in blue prison uniforms.
All the defendants are charged with murder, kidnapping, complicity in incitement to rape, plunder, sabotage, embezzlement of public funds and acts harmful to national unity.
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Seif has been held in the western town of Zintan since he was arrested by rebels in November 2011. The central authorities in Tripoli have tried without success to negotiate his transfer to the capital.
While postponing the trial, the court did decide to allow Seif, and a number of other prisoners held in the eastern city of Misrata, to appear for trial hearings via video link. It judged that transferring them to Tripoli would pose a security risk.
In May, the ICC rejected Tripoli's request to try him in Libya because of doubts over a fair trial. Tripoli has appealed the decision.
But the ICC last October gave Tripoli the go-ahead to try Senussi inside the country.
Saadi Gaddafi, another of the slain dictator's sons, was extradited from Niger in March, and is also due to go on trial. However, he has not yet been formally charged.
Human Rights Watch has called on authorities to grant the defendants full access to a lawyer, adequate time to prepare their defence and the ability to challenge evidence presented against them.
"Putting Gaddafi-era officials on trial without fair-trial guarantees shouldn't leave anyone satisfied that justice is being done," he added.