The mechanism of photo-aligning (PA) of azodye molecules is investigated. We show that PA takes place due to the pure reorientation of the molecular absorption oscillators perpendicular to the UV-light polarization. The model of the rotational diffusion of the azodye molecules in the field of polarized UV-light was used to explain the formation of the photo-induced order in azodye layers. We constructed a special experimental set-up, based on a photo-elastic modulator, that allows to make accurate measurements of the phase retardation δ of thin film as a function of the exposure time texp and exposure power W (W/cm2). The optical anisotropy was induced by a pump beam of Ar+ laser (λ=457 nm). Fitting the experimental curves d(texp) for different power values W, we can estimate the coefficient of rotational diffusion D and azodye order parameter S(texp).