To solve the problems of nonlinearity in fullenvelope flight of UAV and control surface′s efficiency varieties caused by battle damages or surface failures and time varying disturbances in flight environment, two pitch attitude control laws which are L1 adaptive controller and PI controller have been designed for comparatively analyzing control performance. A series of simulation experiments with model uncertainties, control surface′s efficiency varieties and various disturbances are developed and simulation results show that L1adaptive controller is obviously better than PI controller in stabilization and robustness, which meets the control requirements of UAV totally and lays the foundations for engineering application.