Experimental investigations of modelscale coaxial rotors in hover and in forwa rd flight are carried out in water tunnel by particle image velocimetry (PIV) te chnique. The distributions of instantaneous vorticity and velocity of the rotor flow field are obtained. At the same time, the trajectory of the tipvortex, th e wake contractions in hover and in forward flight, and the radial distribution of induced velocity are measured. The wake geometries and aerodynamic interactio n characteristics of coaxialrotor flow field are also investigated. Upwash hap pens at the outboard edge of the blade tip of the lower rotor, while no such phe nomenon happens near the upper rotor blade tip. From comparisons with the prior performance experiments, it can also be found that mutual interferences are play ing a positive role in the coaxial rotor configuration when hovering. Furthermor e, the tip vortex trajectories of the single rotor and coaxial rotor are achieve d during the experiment, in hover and at different forward flight ratios, respec tively.