Refractive indices of two carboxylate nematic liquid crystals have been measured throughout the nematic and isotropic phases to determine their order parameters by using the Vuks method [Vuks, M. F. (1966). Opt. Spektrosk., 20, 644], and characterization of anisotropic behaviors. These factors characterize anisotropic behaviors of selected liquid crystals with negative dielectric anisotropy that are essential for their applications and improvement. The temperature dependence of refractive indices, birefringence, and order parameters in the anisotropic phase have been investigated for these nematic liquid crystals. Furthermore, the alkyl substituents' effects on the order parameters of carboxylate liquid crystals were also investigated.