Air France has estimated that last month's strike will cost it about €500m (£393m) in lost revenue, reports the BBC.
The struggling French flag carrier was in dispute with pilots over plans to expand its budget subsidiary Transavia.
Finance director Pierre-Francois Riolacci said there would be an immediate impact of about 320-350m euros, plus loss of future business.
The Air France-KLM group, Europe's second-biggest airline, had already cut its profit target for 2014.
Air France said that total passenger traffic fell 15.9% in September, adding that bookings for the next three months were also down.
The airline put the expansion of its Transavia budget carrier on hold after the two sides agreed to talks.
Read more of this report from the BBC.