Abstract: The resonant frequencies and corresponding mode shapes ofsteel thin-wall cylinder shell samples each containing an obliquecrack are obtained by a laser holographic interferometry method.It is shown that the oblique cracks have a greateffect upon the vibration characteristics of the shells, with the mode shapes ofthe shell being changed very much and looking very complicated. A kind ofvibration appeared only in the local area round the crack which iscalled the partial vibration by this paper is discovered. The partialvibration, which is responsible for the complicated mode shapes, has resonant frequencies andmodes of vibration of its own. The vibration appeared in the entire shell iscalled the original whole vibration. In a similar way, the original wholevibration has the resonant frequencies and modes of vibration of its own.When the resonant frequencies of the partial vibration close to that of theoriginal whole vibration, the two kinds of vibrations will come together. Sothere are three kinds of vibration mode shapes: pure partial vibration modeshapes, pure original whole vibration mode shapes and the intercouplingvibration mode shapes in which partial vibration and original vibrationinteract with each other. The longer the crack is, the lower the resonantfrequencies of the pure partial vibration and pure original whole vibrationare, but the resonant frequencies of the intercoupling vibrationoccasionallyshows an inverse tendency, which is a phenomenon which may be confusedwith two kinds of the resonant frequencies.