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≤80.0nm, the average area of the quasi-circular aggregates, namely S av , satisfies S av ∝(1−e −h/H ) with H=27±3nm; for h>80.0nm, S av increases quickly with h. The interpretation for this phenomenon is also presented.