To analyze the influence of structure of the cellulose derivatives on the mesomorphic behavior it was carried out the investigation of their molecular properties in dilute solutions, Flow birefringence and intrinsic viscosity of cellulose derivatives were studied. The optical anisotropy and equilibrium rigidity of polymer chain were determined. The investigated cellulose derivatives are characterized by the high equilibrium rigidity. It was established that their molecules have the conformation of a kinetically rigid coil. It was shown that the values and sign of flow birefringence and correspondingly the optical anisotropy of monomer units depends on the structure of side substitutes and are influenced by the solvent nature and thermodynamical quality of polymer solution.