An unmanned aerial vehicle is developed and the wind tunnel test is carried out to illustrate the basic principle of circular wing servo-mechanism. Test results show that the circular wing (rounded wing) rotating around its center, can twirl therewith due to the imbalance of aerodynamic resistance from both sides, thus it eliminates the sliding effect of wing surface. Aircraft with this kind of aerodynamic structure possesses head mobility without leaning wing when it rotates around its vertical axis. Circular wing is symmetrically relative to its axis. During the nose deviation from route, circular wing, sharing the same axis with fuselage, rotates to the opposite direction under the aerodynamic resistance, to enter the course in new longitudinal axis and maintain the aerodynamic balance of wings.