Studies of the mechanical and optical properties of undrawn polypropylene fibres by annealing and drawing were performed. The optical properties and strain produced in polypropylene fibres at different conditions were measured interferometrically at room temperature. It was found that as the draw ratio of the fibre increased, its birefringence, Delta n(a), increased at a constant rate, and then nearly levelled off. The refractive index, n(parallel to), and polarizability, p(parallel to), increased with different draw ratios; but for fibres annealed at 70 and 100 degrees C, there were no acceptable variations. For fibres annealed at 130 degrees C, n(parallel to) and p(parallel to) increased compared to those fibres annealed at 70 and 100 degrees C. An empirical formula has been suggested to explain the relationship between the cross-sectional area of polypropylene fibres with the draw ratio, and the constants of this formula have been determined. The effect of annealing on the refractive index profile of undrawn polypropylene fibres, before and after thermal treatment, was studied. The strain optical coefficient and the Poisson’s ratio were calculated over different draw ratios. The results obtained clarify the effect of annealing time and temperature with different draw ratios on the optical behaviour of polypropylene fibres. Microinterferograms are given for illustration.