[en] A method is proposed for investigating the refractive indices and birefringence of anisotropic fibres using transport intensity equation (TIE) technique. The method depends on utilizing the matching and mismatching between refractive indices of fibres and their immersion liquids. In which, when an anisotropic fibre immersed in a liquid of refractive index matches with its parallel or perpendicular refractive index, the focus position is obtained at a definite angle of the polarizer. This angle gives the polarization state in the selected direction. The second polarization state position can be obtained by rotating the polarizer 90° from the first position. The Canny edge detector software was used to detect the best focus position at each direction of the polarized light. The optical system of the TIE technique was modified and calibrated, with the aid of the proposed method, to differentiate between the polarizations directions of anisotropic fibre from recording intensity image. Poly (ethylene terephthalate) highly oriented fibre was used as a reference sample to calibrate the optical system. The proposed method was applied to investigate refractive indices and birefringence profiles of isotropic polypropylene (PP) fibre, low birefringent PP fibres and anisotropic nylon-6 fibre. The results of the method for different types of fibres show a good agreement with the published data for the same fibres.