We present an experimental investigation into the strain-induced crystalline microstructure, under biaxial elongation in Polyethylene terephthalate (PET). The aim of our study is to achieve both mechanical tests representative from the blow-moulding process, and microstructural measurements. We therefore examine 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 crystallites. PET injection-moulded thick specimens were subjected to bi-axial elongation tests, both equibiaxial and sequential, using several elongation speeds, draw ratios and temperatures (above and close to T-g). The strain field was determined using a home-developed image correlation technique that allowed us to identify all the strain components at each point of the specimen, even when the strain field was not homogeneous. After the completion of the strain path, the specimens were either quenched at room temperature or cooled down very quickly using liquid nitrogen, in order to dissociate the contribution of stretching from that of relaxation. Microstructure observations were achieved by means of differential densimetry and WAXD (Wide Angle X-ray Diffraction) using a synchrotron beam. The results obtained confirm the strong link between the stress tensor and the crystals orientation and the crucial influence of molecular orientation on final microstructure (crystalinity ratio, crystals morphology).