The investigation of the surface anchoring energy for nematic liquid crystals has always been an important branch of liquid crystal physics. By measuring the twist angle in the twist cell and the surface torsional angle between the rubbing direction and the liquid crystal director at the substrate boundary, an alternative measuring method of the surface torsional anchoring energy for pure nematic liquid crystals without chiral materials is given. The torsional anchoring energy of the interface between 4-n-pentyl-4'-cyanobiphenyl and rubbed polyimide is determined.