Abstract: The experimental study shows that the characteristics of the boundary layers including both the unseparated and separated can be improved by the air jet vortex generator and its effect is quite comparable to that of the conventional vortex generator. The traces and strength of vortices behind the jet have been found by fluorescent thread method at different jet velocity ratios and lateral injectioa angles. A pair of the vortices is formed behind the jet and moved upwards by the entraining action of the jet. At lateral injection angles other than 90 degrees the two vortices differ one from another in strength. The vortex on the acute angle side fades away very soon downstream of the main flow, while the pair on the obtuse side is left acting. The strength of the vortices and the height of the trace are both increased with higher jet velocity ratio and larger jet diameter respectively. It is found that the vortex strength achieves the maximum when the lateral angle of injection is about 30 degrees. A model for mechanism of the vortex formation with an air jet vortex generator is also suggested on the basis of observatioa through visualization by dye in water tunnel.