A dynamic method was Suggested to determine the variation of both refractive index and transverse sectional area of regular and/or irregular fibers during the dynamic cold drawing process. The suggested method depends on the physical principle of constant volume of the filament during the cold drawing process. The mathematical analysis of the method was given. The optical and geometrical properties of acrylic fibers with denier 2 and polypropylene c fibers were determined. One dimension optical Fourier transform was used for automatic analysis of microinterferograms. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 112: 1909-1915, 2009