Abstract: The current rotor rig for single rotor test is modified for the experiment of tandem twin rotors. By the PIV technique, the measurements on the rotor blade tip vortices for tandem rotors in both hover and forward flight have been carried out. The interactions between the twin rotors and their influences on blade tip vortices are investigated by altering the longitudinal and axial distances between the twin rotors and thus changing the overlapping area. It is shown that the axial displacement of the tip vortex gradually increases with the longitudinal distances between the two rotors in hover, but the change of radial displacement of the tip vortex is not the same, and it reaches the minimum value when the longitudinal distance is about equal to 1.8R. On the other hand, when the axial distance between the two rotors is modified, the tip vortex displacement in axial and radial directions are also changed, but the changs are smaller.