The crystallization of poly(ethylene terephthalate) (PET) under tensile strain was investigated using wide angle X-ray diffraction. Real-time investigation of the crystallization state, including the crystalline ratio and the crystallite orientation, of the material could be undertaken due to the high brilliance of the synchrotron X-ray source used in our study. Initially amorphous PET specimens were stretched at different strain rates and draw ratios at the same temperature (above and close to T-g). Our experimental set-up was designed to undertake simultaneous recording of the X-ray diffraction patterns and the mechanical parameters. Up to the draw ratio of 500% and draw rate of 0.75 s(-1), the crystalline development dynamics corresponded to three different regimes. (i) For small enough extension rates, there was no measurable crystallinity during the drawing process. The crystallization developed after cessation of deformation. (ii) For intermediate extension rates, the whole crystallization process took place during the deformation. (iii) For the highest extension rate involved in our experiments, the crystallization started during the drawing process and continued after cessation of deformation. The mechanical behaviour of the polymer was simultaneously recorded and correlated with the induced crystalline microstructure. In particular, we were able to discriminate the influence of crystallite orientation and crystallization growth on the mechanical behaviour of the material.