The Optothermomechanical device (OTM) attached with the double-beam (Pluta) interference microscope was used to develop it to study the structure deformation of polypropylene fibres stretched at high draw ratios. The cold drawing mechanism was done in two cases, the case of fast and slow drawing. The OTM device enables us to correlate both the mechanical and thermal properties of polypropylene fibres with its optical properties. In fast drawing mechanism, the microcracks deformation was detected at high draw ratios. It increases with increasing the temperature and draw ratio. In slow drawing, smooth deformation was detected. The influence of temperature on the refractive indices and their profiles of polypropylene fibres are studied, at different draw ratios, in the two cases. Microinterferograms are given for illustrations.