In this paper, a flight dynamics model of a tiltrotor aircraft is developed used basic equations of motion. This model focuses on control aspect of tiltrotor and is able to show inherent tiltrotor characteristics; then a flight control system is designed used novel L1 adaptive control theory. The XV-15 tilt rotor aircraft is taken as an example to verify this modeling method and flight control system; finally, numerical simulations are presented to show the performance and robustness of the proposed controller.