Nouri Al-Abbar told The Associated Press that the delay from the original date of June 19 also gives the commission more time to look into appeals regarding the qualification of candidates.
The assembly will write a new constitution and form a new government.
The election will be Libya's first national vote since the capture and killing of longtime leader Moammar Gadhafi last October.
The election commission's website says nearly 2.8 million Libyans are registered to vote. It says that more than 2,600 candidates and 370 parties have also registered to run in the election.