Aiming at altitude-keeping of a tilt tri-rotor UAV in flight mode conversion,a new transition strategy was proposed,by which the UAV could implement the transition directly from hovering state without requiring an initial velocity.The Newton-Euler method was adopted for dynamics modeling of the transition mode.The relationship between tilting angle and flight speed,i.e.,the corridor curve of transition mode,was obtained according to the model analysis.Flight test was made to two different tilting modes by using the experimental prototype of tilt tri-rotor UAV.Analysis to the telemetering data obtained from ground station shows that the corridor curve of transition mode is effective for altitude-keeping.