Polarized components of the fluorescence emitted from rod-like molecules (probes) oriented in a polymer medium are analysed theoretically. An ’envelope curve’ of the polarized fluorescence intensity obtained in a rotating frame of two polaroids in the measuring system, is useful for characterizing the molecular orientation distribution. The model calculations are carried out in terms of three basic molecular orientation modes, i.e. uniaxial, random, and planar orientation modes. Generally, the envelope curve is affected by the birefringence due to the optical anisotropy of the polymer medium. In the highly uniaxial orientation mode, however, the shapes of the envelopes are scarcely distorted by the effect of birefringence. The fluorescence technique to measure envelope curves is employed to investigate the structural change of nylon 6 fibres upon drawing. The transformation of molecular orientation pattern in the non-crystalline phase of the drawn fibres is described satisfactorily by using a combination model composed of the three basic modes mentioned above, with their fractional parameters. A considerable portion of molecules in the amorphous regions may orient perpendicular to the draw direction, before the uniaxial orientation mode becomes predominant above ca 100% elongation.