A water-jet flagellator, having the maximum jet speed of 100 kt, was made to evaluate vibration fatigue life of materials for the SES flexible seal. With this device, the fatigue life was investigated on ten conventional Fabric Reinforced Rubber materials. This flagellation test worked very well as a ranking method of the candidate materials, and it was revealed that two materials, NBR (PVC) on plain weave nylon and NR/SBR blend on twill nylon, gave good fatigue life among the materials supplied to the test.