The phase diagram of binary system of 4-n-pentyl 4’-cyanobiphenyl (5CB) and 4-pentyl phenyl trans-4 pentyl cyclohexane carboxylate (D55) is presented, which shows the presence of an injected smectic-A phase and a higher order smectic phase (SmG). Refractive indices (n(o),n(e)) and densities for different compositions of this mixture and for one pure compound (D55) are measured. The orientational order parameters are determined from the polarizability values (alpha(o),alpha(e)) which are calculated using both Vuks’ as well as Neugebauer’s internal held models. The experimental order parameter values for the samples having smectic A phase have been compared with McMillan’s theory and with Maier-Saupe theory for the nematic cases.