The transmission characteristics of the red He-Ne laser light beam, with wavelength lamta = 632.8 nm, by the nanomagnetic granular thin films Co-Al2O3, with Co components of 20, 30 and 50 at percent, were investigated under the effect of external magnetic field applied perpendicularly to the plane of the thin films, with intensity varied from 0 to over 400 Gs. The results showed that for the Iow-Co thin films contained 20 and 30 at percent cobalt, to which superparamagnetic behaviour is dominant, an increases with increasesing magnetic field intensity of the transmission coefficient was observed
while for the high-Co thin film, contains 50 at percent cobalt, to which ferromagnetic behaviour saves a salient portion, an opposite trend was recorded. The reasons of this dependence are discussed.