Abstract: An analysis of flight test results was conducted to explain the effects of winglets on aerodynamic load, control-stability and performances. Also, some considerations of designing winglets were recommended.The results of flight test indicated that the increase in maximum lift-to-drag ratio in cruise was about 5-11%[1]] without adverse effects on the lateral stability aud aileron effeciency, etc.A comparison between flight tests and predicted values based on lifting-surface theory was made. Most of them were consistent.