By using STM, the surface structures of different cobalt coverage grown on √3 × √3-Ag/Ge(111) surface investigated. We found that two kinds of structures appear on the cobalt films, one is √3 × √3 named Structure I, and the other is 2 × 2 named Structure II. Structure I is the structure of the islands with the height under 2 atomic levels, whereas the islands with other heights are Structure II. When the coverage of Co was increased, the transformation of structures reveals a two-stage evolution. The percentage of the Structure II increases rapidly at the coverage of about 1ML and 3 ML. However, the methods for these two elevations are different. The first stage of increasing results from sufficiency of cobalt atoms to form Structure II islands, but the second stage is due to the limitation of cobalt covered areas. The cobalt covered areas reach the limitation of 50% at the coverage of 3.5 ML. Moreover, for the cobalt island of 3.7 nm height, the configuration on the top still exhibits Structure II. This result manifests that large amount of cobalt atoms do not transform into its bulk structure, instead, those cobalt atoms maintain the same structure with the bottom 2 × 2 layers. This coverage-dependent substitution of structures indicates that the transformation of cobalt islands is not due to phase transition. Besides, the exchange between cobalt atoms and the bottom silver layer can also be denied.