A silver film system, deposited on ionic liquid substrates by thermal evaporation method, has been fabricated and its structure as well as formation mechanism has been studied. A coexistent phenomenon of quasi-circular and ramified aggregations is observed and both of them are composed of nanograins with the size of the order of 10(2) nm, which can be explained in terms of the different diffusion behaviors of the grains during and after deposition. For the nominal film thickness h <= 80.0 nm, the average area of the quasi-circular aggregates, namely S-av, satisfies S-av proportional to(1-e(-h/H)) with H = 27 +/- 3 nm; for h > 80.0 nm, S-av increases quickly with h. The interpretation for this phenomenon is also presented. (C) 2011 Published by Elsevier B.V.