The Orient Queen can hold more than 1,000 passengers, but it would likely travel less than half-full, officials said.
Some 15,000 Americans have evacuated Lebanon since fighting began 17 days ago. U.S. officials had said their operation ended Wednesday, but then announced Thursday that the Orient Queen would return for more Americans.
Officials said they decided on the journey as the ship was still under contract with the U.S.
Evacuees were being processed at a beach near Beirut's port Friday morning, then bused to a dock to board the Orient Queen later Friday. It was scheduled to depart by Friday evening.
The U.S. embassy in Beirut was in contact with Americans stranded in south Lebanon, and urged them to await "further guidance," an embassy statement said Thursday.
The embassy was evaluating options for getting those Americans to safety, the official said.
Tens of thousands of foreigners have left Lebanon so far, the AP reports.
Canadian officials said they had evacuated a total of about 11,500 of their nationals, including more than 1,700 on four ships Thursday. More Canadian ships were headed to Beirut on Friday.