The geometric structures and stability of Na+Arn clusters were studied with ab initio molecular orbital methods. The clusters of n = 6, 8, and 10 have a high symmetry of O-h, D-4d, and D-4d, respectively. On the other hand, the clusters of n = 3 and 4 are deformed from the expected high symmetric configuration. It is the attractive force between rare gas atoms that breaks the symmetry. The many-body terms also play an important role in determining the detailed structures. The size dependence of the calculated thermochemical parameters are consistent with the reported mass spectral pattern which shows the first sequence of the magic number at n = 6 and 8.