Indium nanofilms composed of ultrafine grains have many applications in photovoltaic technology, microelectronics industry, grayscale lithography and so on. It is unquestionable that ultrafine grains have an important influence on physical properties of the films. For investigating the role of grain size in optical and electric properties of the films, we took In as an example to deeply study the relationship between grain sizes and the optical density (OD) spectra as well as the square resistance in the In, In2O3 and ITO nanofilms with ultrafine grains, which were prepared by multiple-oxidation fabrication technique for stopping nucleation-growth. The experimental results show that In films with ultrafine grains have a high OD value in the range from 350 to 700 nm, In2O3 films, nevertheless, have a smaller OD in the visible and NUV region; while the grain size can greatly improve the conductivity of In2O3 and ITO film but has less effect on that of the indium films. These results demonstrate that the grain size of the film is closely related to its optical and electric properties, and indicate ultrafine-grained In nanofilm, meanwhile, can be used in fabrication of grayscale masks and transparent conductive films due to its good optical and electric properties. Moreover, the morphology as well as composition and chemical bonding states of In nanofilms are analyzed, and the controllability of ultrafine grains is deeply investigated theoretically. (C) 2014 Published by Elsevier B.V.