The oriented crystalline structure, mechanical properties, and stress relaxation behaviors of biaxially oriented PET films were studied. The crystallite orientation factors and crystallite sizes of biaxially oriented PET films with four directions indicating p=0$^{circ}$(transverse direction), p=45$^{circ}$, p=90$^{circ}$(machine direction), and p=135$^{circ}$ were determined by x-ray diffraction. The stress relaxation behaviors of biaxially oriented PET films with machine direction and transverse direction were investigated under strains of 1.0~7.5% at temperatures ranging from room temperature at 13$0^{circ}C$. The following results were obtained. The crystal orientation mechanism was uniplanar orientation type, that is to say, the (100) plane of crystals runs parallel to the film plane and c-axis of crystallites is randomly oriented within the film plane. Crystallite orientation factor and Mechanical properties were made in different sizes with four directions. Mechanical properties fitted in with crystallite orientation factor. The more strain decreased, the greater relaxation modulus was and the rapider relaxation rate was. Relaxation modulus was greater in the machine direction than in the transverse direction and relaxation rate was rapider in the transverse direction than in the machine direction.