Film casting and biaxial stretching of a series of polyester thermoplastic elastomers (TPEs) were studied. Biaxial orientation in the stretched films was characterized by wide-angle X-ray diffraction and birefringence measurements. Biaxial orientation factors were determined. The X-ray diffraction and birefringence clearly indicated the development of planar biaxial orientation in the stretched films with biaxial stretching. The phenyl groups in the stretched PET and TPE films gradually became more parallel to the film surfaces with increasing biaxial orientation. The lower the PET content in the stretched TPE films, the lower the planar bia;dal orientation achieved. The beta form of crystalline PET was found only in the stretched PET films, but not in the TPE films.