Airline fleet composition, which means determining the airplane types and the numbers of each type needed by an airline, is important in affecting its operational performance. Through analyzing the disadvantage of traditional method of evaluating fleet composition, which is difficult to evaluate the revenue and operating cost, we put forward a new method to evaluate the feet composition by integrating the fleet composition and fleet assignment. Under an expected airline operational environment, getting the information of flight schedule, expected reservation demand and mean price of each flight from simulating airline flight assignment, we simulate the airline operation with different fleet composition scenarios, and then evaluate the fleet composition by calculating the corresponding planned direct cost (POC). The optimal scenario is the one with the lowest POC. Numerical experiment shows that this idea is effective.