A molecular model based upon the random walk theory is applied to calculating the rotational self-diffusion (RSD) coefficient and the set of relaxation times of a polar organic liquid crystal (LC) film on the solid substrate. The RSD coefficient is presented as explicit function of temperature, of the molecular moment of inertia and the set of the order parameters (OPs) of the LC film. Both the RSD coefficient and these relaxation times, for 4-n-pentyl-4'-cyanobiphenyl film evaporated on Kapton-type polyimide substrate, using the set of the OPs SL (L = 1, 2, 3, 4), have been calculated. The first three orientational OPs were measured by means of the optical second harmonic generation and polarized absorption techniques, whereas the last OP was calculated using the functional dependence of S-4 and S-2. (C) 2008 Elsevier B.V. All rights reserved.