The resonant frequencies and corresponding mode shapes of steel thin-wall cylinder shell samples each containing an oblique crack are obtained by a laser holographic interferometry method. It is shown that the oblique cracks have a great effect upon the vibration characteristics of the shells, with the mode shapes of the shell being changed very much and looking very complicated. A kind of vibration appeared only in the local area round the crack which is called the partial vibration by this paper is discovered. The partial vibration, which is responsible for the complicated mode shapes, has resonant frequencies and modes of vibration of its own. The vibration appeared in the entire shell is called the original whole vibration. In a similar way, the original whole vibration has the resonant frequencies and modes of vibration of its own. When the resonant frequencies of the partial vibration close to that of the original whole vibration, the two kinds of vibrations will come together. So there are three kinds of vibration mode shapes: pure partial vibration mode shapes, pure original whole vibration mode shapes and the intercoupling vibration mode shapes in which partial vibration and original vibration interact with each other. The longer the crack is, the lower the resonant frequencies of the pure partial vibration and pure original whole vibration are, but the resonant frequencies of the intercoupling vibration occasionally shows an inverse tendency, which is a phenomenon which may be confused with two kinds of the resonant frequencies.