Different experimental methods are available to determine the order parameter in nematic liquid crystals. However, all these methods are not applicable and useful for the determination of order parameter in liquid crystalline phases other than nematic. In order to determine the order parameter in different liquid crystalline phases, we followed the extrapolation technique used by Kuczynski et al., following the suggestion made by de Gennes viz., that any anisotropic physical quantity may be a measure of orientational ordering in nematic mesophase. The order parameter can be calculated directly from a macroscopic quantity measured in the experiment. Using this idea, the authors calculated the order parameter based on birefringence measurements, which can be applied for all liquid crystalline phases, provided the birefringence data is available in those phases. The authors calculated the order parameter in nematic and smectic-A phases of a homogeneous binary mixture of two liquid crystals, 6O.4 and 4O.6 of N-(p-n-alkoxybenzylidene)-p-n-alkyl anilines, (nO.m) series. The results are discussed in the light of the available data on these binary mixtures and other compounds. In view of direct determination of order parameter, the discussion regarding the local field that a liquid crystal molecule experiences can be omitted.