Mo nano-films of different thicknesses have been prepared by ion beam sputtering. The reflectance and transmittance of Mo nano-films in the range of 310 nm to 1250 nm have been measured by Lambda-900 spectrophotometer. Relationships of the reflectance, transmittance, absorptance to the thickness of Mo nano-films in the wavelengths selected 310 nm, 350 nm, 400 nm, 500 nm, 550 nm, 632 nm, 800 nm and 1200 nm have been discussed. The results showed that the optical properties of Mo nano-films have obvious size effect. There is a point of intersection, which is located in the curves of relationships of the reflectance, transmittance to the thickness of Mo nano-films at the wavelength of 550 nm. The corresponding thickness of this point of intersection can be thought of as the minimal continuous thickness of the growth of metal films from discontinuous to continuous in the optical properties. The topography image of Mo nano-film with thickness of 15.76 nm was obtained by using atomic force microscopy (AFM). This topography image is already the topography image of the continuous film.