This study presents an experimental investigation into the strain-induced crystalline microstructure, under biaxial elongation in poly(ethylene terephthalate) (PET), using wide angle X-ray diffraction (WAXD). We examined how the microstructure of a polymer subjected to a complex strain field evolves in terms of its crystalline ratio, its molecular orientation and the size of its crystallite. PET injection-molded specimens have been subjected to biaxial elongation tests, both equibiaxial and sequential, at different drawing speeds, draw ratios and temperatures above and close to T, The strain field was determined using a home-developed image correlation technique that has allowed us to determine all the strain components at each point of the specimen, even with a non-homogeneous strain field. To minimize the effect of quiescent crystallization, specimens are quickly heated with infrared and the temperature was regulated during the test. At the end of the deformation process, the specimens were quenched to room temperature. Their microstructures were later investigated, using both differential densimetry and WAXD with a synchrotron beam. Influences of strain rate, temperature and strain path sequence on the size of the crystallites and their orientation are evaluated.