A spokesman for the General Attorney's Office in Tripoli said the trial was postponed until May 2 because Gadhafi's attorney was absent, China's state-run news agency Xinhua reported.
The son was in court Thursday when the delay was announced.
Libya's Justice Ministry last week said Gadhafi, former Prime Minister al-Baghdadi Ali al-Mahmoudi and former intelligence chief Abdullah al-Senoussi would be tried as soon as the questioning is completed within the next month.
Libya and the International Criminal Court in The Hague, Netherlands, have been battling over where Gadhafi should be tried. The Libyan authorities have repeatedly rejected requests to hand him over for trial in The Hague.
In 2011, the ICC issued warrants against Senoussi, Moammar Gadhafi and Saif al-Islam Gadhafi on charges of crimes against humanity arising from the regime's crackdown on anti-government protesters.
Saif al-Islam Gadhafi's ICC lawyers argued that he would not receive a fair trial in Libya, but Libyan authorities said he should be tried in the country where he is accused of committing crimes.
Moammar Gadhafi died in October 2011, caught in cross-fire as rebel forces battled in Misurata.