Abstract: A experimental system based on grid projector andhigh-speed camera was established in order to accurately measure 3Dkinematics of fish fin, and the corresponding analysis method was applied. Afree-swimming carp was used in the experiment. Projecting the sinusoidfringes onto its tail fin, the deformed fringe pattern containing 3Dinformation was produced and varied with the tail fin movement. Thesetime-sequence images were captured by a high speed camera. By Fouriertransform, filter, inverse Fourier transform and 3D phase unwrap, thecomplex 3D movement of tail fin was really reconstructed. And then themovement characteristics and the kinematics parameters of tail fin werestudied. Analysis of tail kinematics suggests that, when the swimming speedof carp is 0.5Ls^-1 (L is the length of fish body), the tail beatfrequency is about 1.42Hz. The tail movement mainly occurred in the lateraldimension as the tail beat from side to side. The 3D profile of tail variedduring the tail beat cycle. The tail underwent a cyclic vertical expansionand compression. A dorsal-ventral asymmetry was present in the lateralmovements of the tail in which the maximum lateral excursion of the dorsaltail tip underwent a 15.6% greater than did the ventral tail tip. Thepresent study provides a new method to quantify the analysis of kineticcharacteristic of the tail fin during steady swimming.