CdS semiconductor nano-films were grown on ITO glass substrates by means of hemical bath deposition(CBD), with Cd(NO3)2 as Cd ion and(NH2)2CS as S ion sources. The concentration of Cd ions,deposition temperature, deposition time and posttreatment temperature have an impact on the formation of CdS nano-films. UV-vis absorption spectrum and atomic force microscope(AFM)images indicated that the change of concentration and post-treatment temperature may adjust the band-gap of CdS to obtain stable, homogeneous and compact films. Formation mechanism of the crystal nucleus and CdS film was also discussed. Active sites on the surface of ITO are critical to the formation of the crystal nucleus and a uniform and compact CdS nano-film. The active site and crystal nucleus are formed due to the comprehensive effect of electricity, thermodynamics and chemistry.