We have proposed a new optical method for measuring the twist angle of twisted nematic cells. The method is applied to estimate the azimuthal anchoring energy of a nematic liquid crystal 5CB aligned on the as-stacked polyimide Langmuir–Blodgett (PI-LB) films. The measured values of the anchoring energies are shown to range from 2 × 10−6J/m2to 1 × 10−5J/m2, which depend on the LB deposition conditions and show the optimum condition for the deposition. The values of the azimuthal anchoring energy are known to be one or two orders of magnitude lower than the corresponding polar anchoring energy.