Particle Image Velocimetry(PIV) technique was applied to water tunnel experiment for the flow field of hovering coaxial-rotor.Vortex contours and velocity distribution of the rotor flow fields
configuration and trajectory of the tip vortex were measured.The aerodynamic interference characteristics of hovering coaxial-rotor flow fields were also investigated.Asingle-rotor was also measured for comparison purpose.The experimental results provide reliable reference for aerodynamic calculation and reasonable design of helicopter.