The speed-up of robotic fish is hoped to aim at practical use. It is therefore important to develop methods for making such robots swim faster, and computer simulations are invaluable. In this study, a computational simulation model was developed for three-dimensional fluid-structure interaction analysis. The flow around the caudal fin can be analyzed by treating the fin as an elastic body. Experiments were also carried out and good agreement was found between the experimental results and those of the numerical analysis. Moreover, the effects of the flexibility of caudal fin on the robot's propulsive performance were investigated using computational simulations.