A modified heating device connected to a wedge interferometer is designed to study the influence of temperature on the optical and structural properties along the diameter of undrawn polypropylene fibres. To determine the optical orientation function profile, a model that divides the circular cross section into very small circular zones is suggested. The refractive indices and optical orientation function of each zone are considered assuming there is a constant fully oriented birefringence throughout the fibre. This investigation provides useful information about the structural properties of polypropylene fibres and throws light on its opto-thermal behaviour. The investigation is carried out using multiple-beam interference Fizeau fringes in transmission. Microinterferograms are given for illustration.