The polyester film according to the embodiment can maintain its original characteristics even after a plurality of repeated folding by adjusting the deformed dimension as compared with the initial dimension of the film, in particular, by adjusting the ratio between the deformed dimensions in two in-plane directions perpendicular to each other to a specific range, in the cycle of straining and relaxing the film. Accordingly, the polyester film can be applied to a cover of a flexible display device, in particular, a foldable display device to exhibit excellent characteristics.