In this study, we propose a simplification scheme for modeling a complex bellows structure. Using 3-dimensional finite element analysis, vibration modes and natural frequencies are analyzed. The analysis results are compared with those measured by telemetry system of acceleration. It is found that bending mode of vibration can be activated even a low operation frequency and this leads to uneven distribution of stress. The uneven distribution of stress can be a possible cause for the early failure of a bellows with a large diameter.