A polyester film which is excellent in uniform light diffusibility, heat-resistant dimensional stability and luminance and suitable as a substrate of a light-diffusing plate of a liquid crystal display device. The film comprises a polyester containing polyethylene-2,6-naphthalene dicarboxylate as a main component and has a heat shrinkage in two directions perpendicular to each other in the surface of the film of not higher than 0.3% when heat-treated at 105° C. for 30 minutes, a dimensional change rate in two directions perpendicular to each other in the surface of the film under a stress of 0.37 MPa within a temperature range of 30 to 150° C. of not higher than 1.0%, a density of not lower than 1.350 g/cm3, a total light transmittance of not lower than 87%, a haze value of lower than 1.5% and a thickness of 50 to 150 μm.