The present invention provides a polyester film superior in heat resistance, chemical resistance, insulation property and thermal dimensional stability, and suitable for application to fields associated with boiling or retort treatment, which require tenacity, pinhole resistance, bending resistance, bag breakage resistance on dropping, impact resistance and the like, fields requiring thermoforming or vacuum forming, and various uses such as packaging bags for water-containing food, pharmaceutical products and the like.[Solving Means] The polyester film characteristically shows an initial elastic modulus in at least one direction of 2.5-10 GPa, an impact strength of 40-10000 J/mm, a thermal shrinkage in at least one direction at 150.degree. C. of -0.5% to 6%, a haze of 0.001% to 7%, and an absolute value of the difference in the thermal shrinkage between the longitudinal direction and the transverse direction of not more than 1.1%.