Abstract: Fish fin plays an important role in fish swimming as apropeller and a controller. The mechanical property of fish fin is ofessential importance in exploration the source of high efficiency duringfish swimming. In this paper, a study of stress relaxation in Crucian Carpfin ray was carried out. The relaxation time was 300 senconds and the decayof force indicated that the caudal fin behaves as a viscoelastic material.The fractional Zener model was used to fit the experimental data. Theresults are compared with the 3-parameter and 5-parameter linearmodels, indicating the fraction Zener model gives a better fit than the3-parameter model and fits as well as the 5-parameter model but withless parameters.