In this work, the optical properties of nanocrystalline CdS films in the initial state and after ion-plasma treatment have been studied. The chemical bath deposition technique was used to prepare CdS films with thickness 80–115 nm on glass substrates. The ion-plasma treatment was carried out in argon plasma in a high-density low-pressure radio frequency inductively coupled plasma reactor at an argon ion energy of 25 eV for 30–50 s. It has been established  that ion-plasma treatment leads to a decrease in film thickness by 10–15% of the initial one and the formation of new nanostructures on its surface. The results showed that the sizes of coherent scattering regions during plasma  treatment decreased for a series of studied samples from 8.2–10.0 nm to 6.3–7.7 nm. This led to an increase in the band gap energy of the for nanocrystalline CdS films from 2.53–2.78 eV to 2.95–3.11 eV.