The vibrational motion of Ar clusters (Ar_n, n=20 and 30) having isomers in a variety of shapes was simulated by use of the molecular dynamics method and the mode-separation of the breathing vibration from the quadruple spheroidal vibration was investigated. It was found that these modes of highly spherical isomers were almost fully separated from each other, while coupling between these modes were significant in non-spherical isomers. The relation between the cluster shape and the mode-separation was elucidated
