Abstract: Flight delay is an inevitable problem in the area of air transport. Neither the Warsaw Convention nor the China Civil Aviation Law have precise definition of flight delay. Judging from nomal time for departure and reasonable time for arrival, flight delay is that aviation carrier doesn’t carry passengers to the final destination in appointed time. If transport time isn’t appointed, we should judge it from reasonability of transport time. According to the Reasonability Standard, the carrier of reasonable delay is liable for carrying out its subordinate obligation without compensation for loss. In the meantime, carrier of unreasonable delay is not only liable for predictable loss, but also for breach fee independently.