This paper deals with a theoretical analysis and experimental study of combination resonance of the flexible bellowssubjected to oscillating internal fluid pressure excitation. The governing equation of the bellows is expressed with theMathieu's type equation. The parametric instability boundaries of bellows are calculated by the first-order approximationanalysis using perturbation method. As a result, it is clarified that combination resonance occurs in the bellows due to theoscillating internal fluid pressure excitation. And the theoretical results of the parametric instability boundaries agree wellwith the experimental results.