Particle image velocimetry (PIV) technique was applied to the experiment of coaxial-rotor in hover and in forward flight in water tunnel. The laser sheet was positioned for tip vortices shed at zero and 180 degrees azimuths. The rotor rotation and the imaging system were synchronized to acquire PIV images at specified rotor azimuth angles. Instantaneous vorticity and velocity distributions of the rotor flow fields were obtained. The tip vortices were tracked from their generation to convection downstream. The wake contractions in hover and the wake boundary inclination angles in forward flight were measured. The wake geometries and aerodynamic interaction characteristics of coaxial-rotor flow fields were investigated. For comparison, single-rotor flow fields were also measured. The agreement of the experimental results with the literatures indicates PIV is effective to investigate complex flow fields in water tunnel. The valuable results have important meanings to the aerodynamic calculation and reasonable design of coaxial-rotor helicopter.